This title IS a bit misleading I will admit.

Does the message appear when a new person joins your group? If your on farm, you have every right to get rid of said players that have not cleared. However, should you choose to keep them, then it's whatever happens, happens. This could be a bit tougher to do if more than one new player joins, since you won't know who has cleared and who hasn't, but I don't think blocking in the PF is the answer..

Especially in a theme park type MMO as this one. I just don't see devs giving players the ability to lock out folks from the PF. Plus I can see this being abused in some way or another down the line. People do crazy things sometimes when given the means to do so.