My supervisor won't accept that I won't stay in the lab for a PhD
"I am happy to polish the paper for publication as agreed; however, unfortunately, I cannot accept your generous offer. I have already accepted the other job and committed to it." - repeat and rinse. You agreed to the publication, which is fair enough. It is not your job to satisfy your supervisor's wishes outside of that. 

You are perfectly entitled to choose your future. Don't feel bad about that. You have your reasons to leave. Trust your gut feeling and accept you won't change your opinion. Once you accept that you won't change your opinion, it will be easier for you also radiate to your (former) supervisor that you won't change it.

Do not budge. Repeat your formulation. Only when they realise that you are not going to change your tune, they will relent.