Cadlock (now Cadvault) costs $300 ($1,500 site license). For a free solution, see my reply on thread555-131901 (Is there a way to "lock" a dwg file so it can't be modified?)
It is easy to do, but it's still a little known trick.
FYI, I have used this in the past to send a dwg to clients. You can print it, but you can't modify the entities.
Flores