Many Administrators on account being created after deleted

We have many different administrators on our account with unknown users. The users keep appearing after we delete them. We have seen spam text advertisements published on pages. When we delete those unknown administrators some of our pages (which they did not publish) were found in the trash. When we restored these pages all the images were gone. Does anyone know what can be done to remedy this hack?
One of the administrators email addresses is useradmin@gmail.com. we also found unknown Russian language in our code. When we google this email address the same language comes up. Definitely spam. Can anyone help?