Almost every ToS I've seen has some sort of rule about spam and how it can get your account terminated. As others have stated, it's a good idea to look into the issue yourself first and then contact the client. If the client seems to be truthful yet there is proof that spam is being sent out, it could be that someone hacked their account which is quite common so in that case, your priority should be helping the client secure their account once again to prevent the spam problem from persisting.