There are three main categories of squirrel repellent: chemical, electronic and natural. The effectiveness of each of these methods is somewhat disputed, depending on the number of squirrels and the way in which the repellents are used.
"Natural" squirrel repellents include the urine of predators, such as foxes and coyotes, as well as garlic and other odoriferous materials. According to the website pestkill.org, predator urine is effective, while squirrel-attic.com claims that it is not. The same is true of noise and electromagnet repellent machines, with some sources claiming effectiveness and others ineffectiveness.
According to squirrel-attic.com, traps placed inside an infested attic typically do not work, nor does rat poison. According to the site, the most effective method is to close all of the holes they are using for entry and exit except for the main one. After this is accomplished, installing a one-way door on the main hole should allow the squirrels to leave but not to re-enter. Since they need to leave for food, the attic should be free of squirrels within a few days. After that, it is simply a matter of cleaning and decontaminating the attic and then repairing the main hole to prevent further squirrel access.