Check out in Mae Sai. Walk across the bridge. When buddy wants to see your proof of Vax just ignore him. Pay your 500 bahts at Burma immigration and walk away. He is not going to chase you down. He is so overwhelmed with others, and for $3 a day he doesn't care.
just tell them no. Seriously, tell them no, continue walking. What are they going to do. Pay your 500 bahts. In and out. Just because he is uniformed up means nothing.