You may have a local animal control officer. Your best and probably the most humane bet is to call him or her and see what they can do for you. Otherwise, the strays are on your property and you could round them up and take them to a shelter or service, but this may result in some fees. Glad someone is willing to do the right thing here.
Best of Luck!
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In addition to the recommendations given, I suggest you find out why they are in YOUR yard. Is there a bird feeder that attracts them? Accessible garbage cans? A garage or shed they can shelter in? Something is bringing them to your property; find out what it is and remove the attraction.
You may also want to contact a shelter that does spay/neuter, release programs. Reduce the number of cats and reduce the scope of the problem, especially if it is a feral colony that is overrunning its area.
The foregoing is for general information purposes and does not establish an attorney-client relationship.
As suggested, call animal control. However, as also suggested, you need to figure out why they are congregating in your yard.
This website has lots of great tips: http://www.neighborhoodcats.org/HOW_TO_KEEPING_CATS_OUT_OF_GARDENS_AND_YARDS
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