jplush would take an expieranced Admin or Mod at most ten minutes to work out what was going on, and then ip bannage would be raining down. Not a good idea to really upset larger sites, they may just return the favour.
I tend to join sites with similar sibject matter, use sig links etc (after confirmation that this is okay via the admins), and generally earn some respect from the other sites. If your posts are informative/funny/whatever you will find members signing onto your forums from the other forums. We got three yesterday from a major rugby site that wanted more information than they were getting.
We retain good relationships with the larger sites and are getting some promotion from them.
Better to have some allies from the larger sites than a whole lot of enemies. A lot of subject sites known each other and will get the word out about people spamming etc.
Strangest one we have had so far is a couple of large movie sites already linking up our next proposed site. They know our guys from other forums and are happy to help us out. We are even going to get some news announcements happening from the other sites when we go live.
Generally if you talk to people running large sites which are in the same general area as your own, they will be happy to do something for you. We haven't gone live with our rugby site yet but have forum redirects happening from some of the S14 rugby sites already. Of course we will return the favour.