I am quite often offered things I don't want at that moment, by people offering 'transport' on the street or wanting me to buy sarongs.
As saying 'no' constantly doesn't feel good to me, I just smile and say 'thank-you' - and move on.
They get the message that I don't want what they are offering - but my 'thank-you' at least lets me graciously acknowledge that something was offered, without the feeling of constantly rejecting everything!
~I suggest you do the same - smile, say thank-you, but then don't take it.
Acknowledge the offer. You can nod a bit with a downward motion to show you want them to put the plate down, as you are smiling.
If they try to foist it on you again, you can smile, nod (by which I mean, gently gesture towards the table surface with your head), and say 'thank-you - maybe later' (hold up a flat palm here if needed) - again with the downward nod.
In this way, hopefully, you can train them, and also, feel gracious, in your response.