First, check under View...Properties for the table you want to work in. You should be able to enter a new error message on the Tables Tab for the Properties under Check Constraint for Tables and Columns. I haven't used this property, but it looks like what you want. Also, under Tools...Security...Database Security, look for the Security Tab and see if Server Logins and Database Users Tabs help you out. Again, I haven't used any of these features yet but it's a place to start. If it doesn't help, I would post a new thread in this forum to ask these questions.
Paul