IN 2005 and 2008 you could use a LOGON TRIGGER and check the application name, but be aware that this can be passed as part of the connection string so your real application can be spoofed.

I don't know of a way to do this in 2000.

Scenarios like this is why I don't believe in granting direct table access. If everything is done with SP's, Views, and UDF's then the users can't do this.