Assuming true that bots comes from free trials, it will be worth of consideration adding one or more servers dedicated to free trials only, and datacenter-wide content as duties or other, restricted to work only between those servers. Sort of a separated datacenter, we may call it.

Then, when the player will choose to pay the subscription, give him/her a free server transfer to the standard, unrestricted servers.

About the friends you can get there, if them don't sub, they don't play anymore after free time expired, and if they sub, you all together can decide where to go with your free ticket to another server. So, no missing friends either, and we all are spared from all free trials bots pestering us. win-win.