I have a small software contracting company.  I'd like to find some
boilerplate clauses I could add to contracts with clients to prevent,
or at least discourage, them from hiring an employee of mine that's
going to be contracted to work with the client.  I imagine this has
got to be a standard contractor or temp agency problem, but I haven't
been able to find any standard clauses to deal with this situation.

THe usual course is to include a non-competitive clause in the
contract with your employees.

You could include imposed fees if they hire one of your employees.
They would be on any temp agency time card.