There are a number of humane ways for dealing with stray cats which will keep them out of your yard, ranging from establishing plants which cats generally do not like to working with your neighbors on a stray cat trapping program. In any case, it is important to remember that while stray cats can be very frustrating to deal with, you should not use extreme measures, as these measures can hurt people's pets and wildlife which lives in the neighborhood.
If you have stray cats in the area, the first step should be preventing more from moving in. If someone in the area feeds the cats, you may want to ask them to stop, explaining that the stray cats are causing problems for all of the neighbors. This will discourage more animals from appearing. Any strays which are currently setting up camp in the area should be trapped, altered, and vaccinated; you may be able to find a local humane association which will trap and relocate the cats for you, or at least provide grant money for altering the cats so that they will not breed.
There are also some steps which you can take to keep stray cats out of your yard and garden; these steps will also discourage neighborhood pets who may be making a nuisance of themselves as well. One way to do this is to make your garden hostile to cats, with no spaces to lounge or go to the bathroom.
You can plant things to discourage cats, such as geranium, pennyroyal, rue, lavender, garlic, lemon thyme, lemon verbena, and other strongly scented plants. You can also lay chicken wire in your flower beds, as cats dislike like walking on wire. In addition to chicken wire, you can use pine cones, nut shells, and short stakes to discourage cats from walking in your yard; cats also don't like walking on coffee grounds and eggshells, both of which happen to make great fertilizer.
Some people find that an automatic sprinkler does the trick when it comes to getting rid of stray cats. Sprinklers can be installed with motion-sensitive detectors which cause the sprinkler to go off when an animal walks by, and they can be moved around so that the animals don't learn to subvert the sprinkler.
When working on getting stray cats out of your yard, it can help to coordinate with the neighbors. Many people dislike stray cats because they are a nuisance and they are often unhealthy; in addition to killing wildlife and ruining gardens, stray cats also pose a health risk to pet cats and other neighborhood pets. Because of this, you may find that your neighbors are ready and willing to help out with making your area hostile for strays.