How do I block uninvited prostitutes from clogging my Chat function in Yahoo mail?

Lately, with the new Yahoo, whenever I signed in, it took a long time to load; Then I discovered it was loading dozens of chat invitations from strange women who I suspect are prostitutes (why should female strangers want to chat with a man?). It took me a while to figure out how to stop these uninvited guests by making myself Offline. What I would like to know is how to remain Online but without those prostitutes clogging up my mail? Anyway, why does Yahoo allow them to spam legitimate users in Chat mode?