1- disable siggies for new members... so they can't advertise there... if you let such features open, like avatar, profilepics and siggies, sure they will use them

2- there are not bots anymore, they are real humans hired in hackers teams to spoil/troll forums they know would make a little noise to advertise them. if your forum is on google, sure they will register and spoil your forum if they have permission to do so, because what they post can be indexed by google, so it make advertisment for them... that's why they are hired by these spam companies... no system whatsoever will protect you from that... the only thing you can have is a team of moderators...