Just delete them - don't even decline, that shows that you have reacted. They may be friends of friends, but they may also just be chancers who send a friend request to every facebook poster they view, and that could be dozens a day, hundreds if they belong to a lot of groups or respond to a lot of pages.
When I first joined Skype, my username was my name plus the last two digits of my year of birth ( I had to add a number.) I received loads of friend requests from blokes who described themselves as "generals in the US army", complete with photos of muscly men in uniform. I decided enough was enough and changed my username to be my name plus my age, which was a much larger number than the previous one. The supply of new friends dried up instantly.
I was telling an ex-forces friend about this recently, and added that the US army must consist of all generals, and no privates. I won't repeat his reply.