friendly cat deterrent

i seem to have a very attractive yard for various neighbourhood cats. for the most part, i never did mind if they came around - my dog kiila loves cats, so she seemed to quite like them to play with. i have never fed them, or even played with them - most are too skittish to come too close to me.
but...now i have a wee kitten who i have started to let come outside with me on a harness...and these cats do not seem to care for my baby aslan (cant even call them strays as many seem well groomed and some have collars on - making me think they are someone's pet - dont get me started on people being bad parents by letting their pets roam at large!)
i have tried to get kiila to chase them away, but they arent afraid of her at all - they seem to sense she is just play chasing and doesnt mean business! i have shooed them, tossed tennis balls in their directions, banged sticks together...all to no avail :(
any suggestions as to how to get them to quit visiting? if i knew they werent diseased or wouldnt hurt my wee boy, i might not be so concerned...but some of them have charged at him and do not appear like they want to play nice. kiila has protected him from them - she stands over him and growls at the other kitty and will chase it away...but for the most part, they dont take her seriously :(
i am soooo frustrated! i hate to chase them away as i LOVE kitties...but it is making taking aslan out not so fun (or safe).

You could take a spray bottle with you whenever you go outside with Aslan and give them a blast of water when the other cats come too close or start acting aggressively (tail swishing, ears back sort of stuff). Make sure your aim is good enough not to hit Aslan by accident!

thanks scm! i was thinking of trying that (or the garden hose on a light spray - as these kitties are pretty bold). i am a pretty good shot ;)
there is one guy who is quite friendly to both kiila and aslan, another that looks more curious than anything, but there are two that just want to (and have tried) to attack both kiila and aslan. kiila can hold her own or outsmart them, but aslan is a different story - he is just a wee baby (and he has NO FEAR - too brave for his own good!).

i am not sure why my yard is so appealing to them. i do have a few trees with oodles of birds and squirrels - but so do my neighbours all around, yet they seem to like my yard.

I have never tried this myself but have heard from others that red hot sauce or orange/citrus rinds around the perimeter of what you want to keep protected may help. I know I have friend who has tried this with her gardens to discourage the use of them as liiter boxes.
Good Luck

you can also try something like ssscat http://www.multivet.net/en/products/ssscat/

thanks marko! i will keep my eyes peeled for that :)