Step::1 Login to your WHMCS.
Step::2 Go to Clients section and click on View/Search Clients.
Step::3 Now select the client from the list.
Step::4 Now jump onto the Profile section of the Client.
Step::5 Search for “Allow Single Sign-On” option and untick it.
Step::6 Click on Save changes.
That's It !Check out the video tutorial to get in-detailed procedure. Read out the blog as well.