Is it possible to create a one way IPsec VPN?

For example, I have created a IPsec VPN from my office m0n0wall, to
one of my clients Netscreen's.  Is there some way I can set it up so
that I can access resources on their LAN, but not let them have them
have access to my LAN?

I have tried creating a few rules to try and prevent them having
access, but haven't had any success.

Has anyone tried this before?  Is this even a possibility?