you know, it's weird.. i've taken care of 5 toms-all unfixed.. 3 were strays 2 were Queen's rooftop cats that a family took in & couldn't take to Oregon, and i've never had that problem. even when i brought in a female, my "bubba" cat who i thought would be all over her, took off instead.. she intimidated him. i was very surprised. Maybe he just had too much respect for my wishes or was afraid of my being angry if he did something wrong.
Kick it out! Intimidate it.. for goodness sakes, chase it and yell at it,, coax it with a broom if needs be.. you must not seem mad enough when you use the water on it. I think that people don't give animals enough credit for their perceptiveness. I'm betting that cat thinks you're milktoast.
When you try being scarier, realize that it might crap, jump around your furniture, and claw it's way up the drapes, and break things.. so be set up and ready.. Also, when your cat sees this, it may be intimidated for a while afterwards too... maybe even offended that you ran off it's buddy,..So try to start at a lower intensity, but you shouldn't have to go thru too many rounds of that in the future.