Is there a plug-in that prevents Towny towns in certain areas?
Solution 1:
Yes, WorldGuard should work fine for protecting areas from towns.
I'll assume you already know how to use WorldGuard. If you are not familiar with it, go through the basics over at the WorldGuard Wiki.
Create a region so it surrounds the area you want to protect. Note the region id.
Type "/region flag [region id] blocked-cmds plot, town, nation"
This will prevent people from claiming areas within the region by blocking the associated commands.
Hope this helps.