Observe and ambush.
We live in an age when unnecessary things are our only necessities. - Oscar Wilde
Best, ambush with you hiding below the grills. Put the curtain type back.
When he tries to reach in to move the curtain, grab his hand like no tomorrow. Then get others to go out and hammer the guy.
This is what i call BO BIAN. cause you live there, you cannot be on your toes 24/7. there are only 2 choices get rid of him of get out of there, cause the HDB design will never change. ppl will forever be peepiing into your house.
Which remind mi of hanging clothes outside is another issue.