How do you tell someone you do not fancy them without offending?

You probably can't but try to do it with love. Keep in mind that if they react badly they are probably overly sensitive. That's not your fault. I was once told that people at church avoided me because I was so needy they did not know where to start. It was a rough time I was terminated from work due to an injury I received at work that gave me a 20 lb weight limit for lifting.Since they had me doing total lifts and transfers they let me go. Illegal but they did it.I went 2 years with 0 income. To avoid being reminded that I am socially innept I would isolate.

Sometimes you have to divorce yourself from some people socially. And you have that right. There is are some friends of family members who I want nothing to do with. i don't approve of the way they talk to their children, thier addictions and the way they use people. I saw a T shirt once I wanted to buy to wear when their around it said "Support Search and Rescue Get Lost."

Keep in mind if you say too little they will seek you out all the time.