Why Won't My Mini Aussie Stop Doing This To The Cats?

Okay so, my Mini Aussie will run after my cats and sniff and lick their butts every time they are on the floor. We've tried to teach him not to but he doesn't listen at all and it makes it worse that I have a new kitten and he won't leave it alone.

Help, I don't understand and I have no idea how to fix it.

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