If I ask a old employee to stop emailing and they say no?

Is there anything I can do other than blocking the address if an old employee refuses to stop sending junk mail / jokes to our employees here at the office? They do now work at a competitor and I do not think there is anything I can do but since they are using work emails to email another business who asked they stop I wasn’t sure if there was anything legally I can do to stop it. (other than blocking 🙂 )