Or prevent newbee's to open new topics untill they have aquired some seniority. Those bot's are pretty straightforward : make a new account, create a new topic, make one single spam post, repeat. If you make them wait for 2 weeks before they are allowed to create a new topic that will probably discurrage the bots, but not so much new human members.
It is clear something needs to be done otherwise this forum will die. When I browse the topics for new posts, I do no longer read the titels of all topics "xyz started"; I only pay attention to "xyz replied". This topic being the exeption since it has such a short and catchy title.