Stray Harassing my Cats!!!

Hi there,
I'm new to the site - desperately in need of advice from you cat lovers out there! I live in Spain and own 3 (normally very happy) cats. As we are permanently in the hot weather and live on a shared community with gardens, we have a system of gates that allows us to keep the windows open and the cats can pass in and out of the house freely. In the last month, there has been a large (un-neutered) male stray stalking our little patch of garden and entering the house. Yesterday we awoke to a loud and violent fight between our alpha male and this cat in our kitchen. Our cats are now constantly on edge and guarding the open doorways all the time. Does anyone have any advice as to how we can keep him away and return to our normal peaceful life?
All advice very gratefully received.
Cheers!,
Susanne

Is there any way that you could let your cats just into the garden but not out all the way, and then keep this stray outside at least, so he can't actually be in your kitchen? I can't even imagine that

You could see if a nearby shelter will take him, no-kill. Do they have those in Spain? Forgive my ignorance.

Get the male neutered. Trap him and find a place that will do it for cheap. I think this is the only way that he is going to relinquish, or just not care so much about the territory. Besides he could have all kinds of diseases.
I was just bitten by the un-neutered male that's harrassing my cats and now I have to find out if he's got rabbies. Not a fun proposition.
Also, there's a product from Ani-mate. It's a cat flap that only allows your cats in and out. Works well for my cats.

Hello Susanne! My cats are indoor/outdoor. In my experience, the only one to stop an aggressive stray is your alpha or tom. My Wawa (and Joji in her younger days) have always been successful driving off domineering strays although I must admit, I ended up visiting the vet a few times.
You can also try catching the stray, have him neutered, and then either adopt him or find him a good home. Others may disagree, but neutered males roaming drive gets limited after the operation.