What breed of cockerel to get??????

Think you just need to secure that enclosure chicken george.

If you get a cockerel, it will fight with your visitor and that's cruel for both birds as they take no prisoners.  Your bird or the visitor could be killed or at best, badly wounded.

If the pest cockerel can't get to your girls he's not so much of a pest then 

He won't stop coming.  He'll just wind up your cockerel if you get one and they will try and get each other through the wire.  I have 2 Serama boys and if 1 is loose in the garden with his girls, you can bet he'll be too preoccupied at the wire of the other's pen to enjoy his freedom