This happens to me too and it drives me nuts. There's bad news, and then there is good news.
The bad news: there is no built-in way to disable the pinch & zoom feature in Safari.
The good news: you
can disable it using a program called MultiClutch, which allows you to assign multitouch actions on a per-application basis. The app is beta, so YMMV:
http://osx.iusethis.com/app/multiclutch
The "fix" was posted by jguluarte over at the MacRumors forum:
http://forums.macrumors.com/showthread.php?t=589218
I've quoted the relevant part below:
1) Download and Install "MultiClutch".
2) Once you have it installed, add Safari to the Application list (it is the plus image in the lower left corner).
3) Click Safari from that list to access Safari settings.
4) Add two new rules (the plus in the lower right corner).
5) Set one to "Zoom In" and one to "Zoom Out"
6) Set a key command sequence that does nothing (I set mine to apple
option+crtl+shift] because I know that doesn't do anything)
7) ...
8) PROFIT!