Dog humps my dog at the dog park

My dog likes to go to the dog park, but he will often get humped by other's dogs (or he'll start humping a dog he is fixated on). My dog is 4 years old and neutered, I thought they would out-grow this behavior? What's a polite way to get other's dogs to get off my dog? I try to monitor mine when he mounts himself on another dog I pull him off and say "leave it" but he'll go right back. Help!
 
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