Either disable touch sounds under Sound Settings... Or if you're rooted you can change it to whatever you want or just disable some specific ones, its in /system/media/audio/UI..
Use Root Explorer or something similar... as I said, the sound is in /system/media/audio/UI... you will see a number of UI sounds, a couple which begin with keypress*** or something like that. Test them to see what is what. If you want to disable them you can just rename them (add .bak to the end of the name, that way its easier to undo it later if you change your mind). If you want to replace them you have to give your new file the exact same name and it must be .ogg format like the ones in there.