Well, there are many things you can do, but keep in mind:
1. If you just compress it or corrupt it by eliminating a single bit (will work as a legit file but will not as a program), change colour tables etc. you will indeed make it impossible to use as software - but people will not know it until after they downloaded it, which means while you fucked them, you also kind of fucked yourself because you did pay for the bandwidth...
2. The dialer idea while good, the dilaer company will either lose its licence - or will simply pay the customers back (more likely) so you will not make $$$
3. What I would do in your place, is try to get a bad reputation with warez people. Since these are executable files, change them to malecious files that cause damage (worms, viruses etc.); When you will damage enough people your IP will be in the warez black list... which is where you want to be
4. Of course, you can be a pro-active sys-admin and actually work hard to find them in time and delete them... They all have the same pattern so just keep looking for them.
5. There is another very effective way for doing that, but if I tell you I will have to kill you.
