* There are many humane ways to keep cats out of an area. One is a "Yard Guard" (or similar electronic device), which emits a high-pitched sound inaudible to humans, but extremely annoying to cats. It will protect a radius of about 200 square feet. For a bigger area, use 2 or more. Since cats develop a "path" they use regularly, they will soon learn to avoid the area.
Too expensive
* Motion-activated water sprinklers will keep cats out of an area. For either the electronic devices or water sprinklers, simply Internet-search "Yard Guard," "electronic (or sonic) repellents," or "motion-activated water sprinkler," and you will find a variety of options.
Too expensive
* Natural outdoor repellents can be applied to a garden or a porch area, e.g, orange-scented sprays may help on porch areas, since cats typically do not like the smell of orange. For gardens, natural fertilizers like dried blood (sounds ghastly, I know) work well. They may have a strong smell at first, so be prepared. But I've seen cats encounter the scent and exit quickly. It has to be reapplied after a rain, but once cats learn to avoid the area, you may not have to reapply as often.
unhealthy and unreliable
* Do make use of animal control services in your area if you can. They will come up with the most humane solution, bearing in mind the needs of the cat and the needs of the community. Don't let the prospect of the cat being destroyed put you off: Often cat control proves to be better for both cat populations and human residents.
notice they use the word destroyed
* The cat could be a lost pet. Put up simple fliers and speak with neighbors. Be sure to ask neighbors if they would be interested in adopting the cat, or fostering it a short while, while you go door to door. Maybe you can solve your problem this way, even if you don't find the owner. Also, list the animal as "found" with all animal agencies in the area. You may find the owner has already reported it as "lost" and is only a phone call away.
if you can catch it
* Check with animal-control authorities. Do they shelter stray cats? Will they pick up stray cats? Do you agree with what they may have to do with the stray cat if they don't find the owner and cannot adopt it? (Some will euthanize the cat; some will find it a barn home; different people might object to either solution, depending on their beliefs.)
right after they teach their pigs to fly
* Get it off the street yourself. Make a quick, free website on Yahoo! or Angelfire, etc., with a photo of the cat and its story. Put an ad in the paper directing readers to the site. Include your email on the website. Attempt to find the cat a home where it will receive veterinary care.
see last comment
* Look in the phone book to find a "No kill" shelter (here it is called "Paws"). They rely on contributions to take care of the overwhelming number of strays or lost beloved family pets.
blah blah blah ....wheres the gun at?
* Look in the newspaper to see if anyone has reported a cat of that description missing; if so, put out food for it and call the owners right away. Does it have a collar?
* Call the SPCA; they will either come out, or make recommendations about what to do.
* Remove any tempting plants or items a cat is attracted to if cats seem especially attracted to your land. If you have a problem with one repetitive cat, just simply take it to a shelter or Animal Control if you don't want to adopt it yourself. Do not just take it in without any adoption papers and a vet checkup!
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Answers.com - How do you humanely get rid of a stray cat