First things first, press your bow and remove the strings. All of the following steps could contaminate or damage your string.

The first thing I would try is some penetrating oil like liquid wrench. Soak the head of the bolt as well as the open end of the hole if visible. Give it at least an hour to soak in, then try to remove it again.

If that does not work, you are down to drilling. If you don't feel comfortable with this, take the offending cam off the bow, and take it to a good bow shop or machine shop. Ask them to drill out the stuck bolt and insert a helicoil. This will restore the threads that were "removed" due to drilling.