Just thinking about various bits I have read on PT...you may have to really show that you are the "Alpha Cat" and take charge.

It sounds like Big Mo gets things started...so a Time Out for Mo when he starts stalking Stitches would work - or the old spray water bottle. If Mo knows what "no" means, use that with him!

Failing all else - perhaps Nancy Efrusy the pet psychic can have a "chat" with Mo.

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Good luck! I hope it's as simple as being a bit firm with Mo! (And rewarding him when he stops?)