How can I prevent coupons from appearing in a site search?
There is a coupon option to change that and make them visible to search.
In 2.2 there were some issues with this function to remove coupons from searches and bbPress, those have been resolved with 2.2.2 and later, but other plugins may have issues, if they make their archives considered search results as bbPress does. This is extremely rare.
If you would like to use your own function to modify the search you can turn off the coupons option and exlude the post type directly: cctor_coupon