You can probably just open it up and put something in there to bind up the gears. You could just fill it with glue, but I've always used a bit of bandage or something like sticky tack so I could undo it later.
This is assuming you can open up the differential, I only have experience with Mini-Zs.
Actually I just googled real quick before submitting the post and found this immediately
http://rc-drift.com/rc-drift-forum/i...p?topic=6270.0