Never done it to a snowboard but you may have to drill out the screws. If you've got a buddy with a drill press it's much easier than using a handheld drill.
If all else fails its the only way to take it apart without butchering the bindings and board.
Main thing is to get a good HSS bit just slightly thinner than the threads (use a screw that you've taken out or any standard binding screw as a guide)... If there are some threads left in the hole you can clean that out with a tap (i.e. from a tap and die set).
Now that I think about it if you don't already have these tools just take it to the board shop you bought it from! Which hacks did it, Mission or Source?
