The Cat Squatter

Our cats have the habit of sittiing on the window sill when they want to come in. However, the one on the right isn't ours. I think he's too fat to be feral, and he comes into the house at every opportunity. I've even caught him sleeping in front of the pellet burner in the kitchen. However he isn't a house cat and poos and wees, even more than ours do so we con't want to encourage him. We chase him off but nothing seems to deter him. We have to keep the catflap shut because of him. Any ideas how to get rid of him, without hurting him?