Creating users with limited access

We host a number of websites for our clients. Recently one client has signed with another IT provider to provide their emails via Microsoft Exchange. The Microsoft supplier now wants access to our control panel to make changes. I am concerned that they could take a copy of the site and move the hosting. Is there a way to lock the file manager section? Or give them limited access? They only need the email and domains sections. I think the new IT provider will persuade our client to swap hosting providers if I don't relent and give them access. I hope you can help.