There are two groups that the Fediverse are very successful in keeping off: conservatives and Blacks. They are kept off in the same way, but most people on the Fediverse only see how one of them happens.
It's impossible to keep a dedicated person away from the Fediverse. If they are kicked off of one server, they go to the next. If they are kicked off of all servers, they start their own. If that server is defederated, they create another.
So what works to keep a group off of the Fediverse? Defederation doesn't work -- it only slightly affects which servers they communicate with.
Attacks keep people off of the Fediverse.
A message positive for Donald Trump will act like a foreign body in a well-functioning immune system. Dozens will race to fight the intruder's ideas.
A racist message won't get the same immune response. Why not? Because most people in the Fediverse won't ever see it.
Defederation is a double-edged sword. It gives a place for the worst people to hide.