I would say let them see to it, but I know if it was me, I chase the other bugger out now and again.

When I tried letting Ribs out a while back, a bad cat from up the road was being a pain, I had to walk around with the garden broom to defend her while she sniffed the garden, Rusty is a sod, he wont bwe chased away, he couldn't give a monkeys!!
Anyway, I gave up in the end and kept her in until her pen was built.