I'd like to offer an alternative to David's comments about nylon hinges (either the flat kind, or the round "Robart" type:

There's a pretty handy bit for a Dremel tool out there that looks like a miniature skill saw blade. It's about 1" in diameter. I found mine at Harbor Freight.

It makes a fairly easy job of cutting nylon or plastic hinges. Just bend the surface down, lay the blade into the gap between surface and wing/stab, and cut through the hinge.

Once that's done, it's a pretty quick job to sand any protruding bits down flush with the wood, and then drill/cut new holes/slots for new hinges next to the old ones.

Certainly nothing wrong with covering around them as David suggested, but I've found that method to be a quick, simple, and safe (to the airplane anyway...*heh*) way to remove nearly any type of 'stubborn" hing that's not easily cut with a hobby knife.