The best solution posted so far is the hose. Cats hate getting wet and usually won't be back after one soaking. The main problem is getting out of the house and to the hose without spooking them. I don't know bout your feral cats, but mine are pretty crafty. Just the sound of a door opening and they're gone.
I saw a motion detector actuated sprinkler at ACE Hardware the other day.
It was advertised as being for keeping deer out of your garden, yard, etc. I wonder if it would be sensitive enough for cats?