How to block email sender using Subject

Hi Community 

I am new here.

We have UTM SG210 firewall, we got an serious issue for one of our client, below is brief description;

Someone is sending SPAM emails (targeting only 1 user) 5-60 every day with the same subject line by using different email servers, so it is not possible for us to block every sending server domain because every time they use new email domain but using same email subject.

Could anyone please advise how we can block such emails by using some keywords or subject or any other possible way?

Really appreciated your help.

Thanks

Gohar