How to prevent or stop my cat spraying...?

My Husband and I are enslaved by two Bengal cats – brother and sister – called Raja and Suki. They are 7 years old and have been with us for 4 years. We were chosen, by them to take them into our home (a long story) and to rescue them from their plight: living in a tiny one-up-one-down cottage, without garden (Cornish mining cottage) alongside two other cats and two small dogs. They were not ill-treated, but obviously were overcrowded.

They are beautiful, intelligent, entertaining… and a damned nuisance. They imitate each other. They rile each other. The are competitive. So, we do our level best to treat them equally, to feed them in the same way (problem here, they don’t eat the same food, unless it’s chicken), to maintain our own place as their superiors… difficult but most of the time we manage. But, Raja sprays.

He is a neutered cat, has a cute little fluff-bag where his bollies once were… but still has a desire/urge/reflex/unconscious need to put his scent in certain places around our home.
Randomly.

It is not in response to his sister… We have identified certain triggers: local un-neutered Toms in the garden (he and Suki are indoor/outdoor cats); visitors to the house that smell of other cats, or occasionally, dogs. That’s understandable.

But, it seems sometimes, he just gets bored and goes about sniffing, then finds an area that needs his scent on it. Urgh. He takes an unholy interest in his own scent… A lot of bottom attention… We are experienced cat-managers, are deeply attentive to these two, but it is proving almost impossible to make a difference to Raja’s unfortunate habits. Advice, comment, jokes all welcome!