It is very likely that there is a way to disassemble your fork. If this is the case, to make it completely rigid you need only replace the springs with similarly sized wooden dowel from the hardware store.

You may be able to use a clamp but this is likely to damage the fork and result in making your fork just work more crappily, not completely limiting its movement.

ALso, you can probably get a good quality rigid fork for ~$40 from the LBS, or find a $20 donor bike at a garage sale or something.

Stiffening a suspension fork is a good way to make a bike better for riding roads or smoother trails.