There are definitely people who don't want to take "no" for an answer. It is probably cultural/regional. Seems to happen more here in Tejas than it did in SoCal.
I had that problem yesterday. Someone brought in something they really loved and wanted others (me included) to try. My "no, thank you" was a personal disappointment to them.
Another tactic is to accept a small portion but don't eat it. People rarely watch to make sure you eat what you take.