stopping personel e-mails

Is it legal? Probably. As long as in an emergency they can receive calls. Like say a family member is hurt or some such, they need to be gotten a hold of.
But the real issue is this. If the staff has the time to send and receive mail, they don't have enough to do and the personal accountablility to make sure it is done. If they do have enough to do and everything is getting done, why are you hounding them about a few personal e-mails? Don't make your company a b**** to work for or you'll lose your quality help.

That said, you can block domains from the e-mail server. How you do that depends on what server you are using. For instance, only e-mail from within your company's own domain is allowed in or some such. Alternately, you can block personal mail domains like hotmail, yahoo, aol, etc.