The U.S. Air Force recently revealed that it is launching four GSAP (Geosynchronous Space Situational Awareness Program) satellites that will spy on the most valuable and potentially dangerous (to the United States) foreign satellites. Not the low flying ones, which can be monitored from the ground, but the more expensive ones that operate in 36,000 kilometer high stationary (geosynchronous) orbits. Up there you have surveillance and communications satellites that basically stay over the same area on earth.
The four GSAP birds are mobile and will move about in the geosynchronous orbits checking out the competition and, in particular, looking for satellites equipped to attack other satellites in that space. They may spur an arms race up there if other countries, particularly China, suspects that the GSAP birds have a secondary attack function to knock out key (especially in wartime) Chinese commercial and military satellites in the geosynchronous region. The stated reason for GSAP is to give the U.S. (and its allies) a better idea of who has what up there and to have that information first. Two of the GSAP birds go up this year and the second two in 2016. Information on GSAP satellites was made public to try and avoid paranoid and unpredictable reactions from nations with satellites up there once they noted the GSAP birds moving about.
Prior to the GSAP program it was believed that the best way to deal with geosynchronous satellites was via jamming or otherwise messing with the signals going to and from these satellites. This led to the greater use of anti-jamming technologies like frequency hopping and DSSS (Direct Sequence Spread Spectrum) on the sending and earth side command stations. But all users of civilian satellites cannot be equipped with these anti-jamming devices. The satellite operators can use this stuff for the control signals (going to and coming from the satellite) and that is increasingly becoming necessary. Another problem with this approach is that jamming protection reduces the amount of data that can be sent, which is a serious and expensive cost for commercial communications satellites.
Meanwhile, the jamming of civilian users grows, usually as part of a state censorship program. For example, in late 2012 Syria and Iran were accused of jamming news service sent to Iran and Syria by BBC, France 24, Deutsche Welle and the Voice of America via radio and satellite. This jamming was apparently in retaliation for European communications satellite operators refusing to continue carrying 19 Iranian TV and radio channels (as part of the growing embargo on Iran) to audiences outside Iran. Syria and Iran denied they were jamming but there was ample evidence that the jamming was coming from those two countries. Over the last decade the U.S. has developed equipment and techniques for locating the source of jamming with considerable accuracy, and that effort has most frequently caught Iran doing what it always denies.
Then there are the increasing number of incidents of space satellites being "hacked". It is believed that this is, for the most part, actually just an increase in the number of satellites up there and the number of ground stations broadcasting information up into the sky. Most of these "hacks" are just satellite signals interfering with one another. Same with cases where people believe their GPS or satellite communications signals are being jammed. On further investigation the real reasons tend to be less interesting and a lot more technical. All this usually has a large element of human error mixed in. But the 2012 problems with satellite reception problems in Iran and Syria appear to be jamming.
But all this accidental jamming only demonstrates how easy it is to do it on purpose and there have been several examples of that. In response the U.S. Air Force, which has taken the lead in developing electronic tools for attacking and defending satellite communications and the satellites themselves. The air force has been training people to use these techniques. This effort involves figuring out new, or improved, ways to jam satellites. Then you keep that stuff secret in case potential enemies have not figured this out themselves. Next, you work on ways to defeat the weapons developed. Most of this is playing around with the signals themselves. You can un-jam a jamming signal with another signal. However, a lot of trial and error is required and you want to get that done way in advance of any actual war. When you do have to use this stuff for real, you have to expect that the enemy may well have come up with some angle you missed. Thus, there will be some rapid improvisation, and you will have more time and resources for this if you have worked out ahead of time the details of disasters you have already anticipated. No one is releasing much information about this, for obvious reasons. There won't be much discussion from any government, unless there is a terrorist attack using these techniques. That's yet another thing to worry about. There have already been such attacks in China, by a banned religious group, and elsewhere. It can be done, it just isn't easy and it's not getting easier.