Say "Thanks for thinking of me. I wish you the best. I'm not interested in catching up."
if they persist, "I'm really not interested though I appreciate the invitation. Take care. Goodbye."

That's if it's by phone or in person. If it's via FB, delete and block them. You could write a post saying you're just doing a cull to keep FB to close friends only before you do, or not.

I think it's great you're protecting yourself like this. Remember, it doesnt matter what other people think of you. It matters what you think of you.

You could also imagine a pool of light around you, protecting you, and ask the universe to only bring positive people into your life.