You might have worn the word "off" out, by saying it several times for one incident at at time.
Also if you have removed him when he has not responded, maybe he expects you to do that now?
Also he's possibly just being a challenging adolescent and pushing the boundaries a bit more, hang on in there and be consistant
We still have to tell Tia "off" at times and she is 17 months, since she has been speyed she has become particularly obsessed with human food, I daren't leave anything close the the edge of the worktop
She does understand "off" though and does respond (unless she thinks you aren't looking
)