Random New users

Since adding the restrict content plugin (maybe?/ I suspect so , but I can’t be sure. ) I have found multiple new random users being added to the user lists. Normally we have about 60 users; we now have 10-20 users being added every day. An example username is [email protected]. We do not allow new users to register themselves; New users are added mainly by me, but there are other administrators on the word press installation.

Any ideas on how to stop these users would be appreciated.
They do not seem to be accessing the system…. but who knows.