Think of man 慢 and mang 忙

BAN means to prohibit something formally or implied prohibition brought about by public opinion or tradition. Eg:There's a BAN on smoking in this country. BANG a loud thumping sound or a blow. Eg: He got a bang on the right side of the head.

When you say "ban" only the tip of your tongue touches the roof of your mouth to the front just behind the teeth... when you say "bang" more of the tongue is flat on the roof, and a little off to the back. Stretch the "a" a little more in "bang". The ng is what we call a palatal sound because of the tongue flat on the roof of the mouth like it is.