"Wait up" is something you say when you are asking someone to wait for you, usually when you're going somewhere else; it's not really the best way to ask someone to stop doing something. You could potentially say "Wait!" to get them to stop temporarily, but it doesn't mean the same thing as "Stop" or "Hold up." If I wanted someone to stop doing something and I said "Wait!" I'd probably feel obliged to follow it up with "Stop!" or "Don't do that yet."
Yes, I think it's also used as a verb, but not an imperative.
He's going to hold up the bank.
Although I think this sounds a bit like you're saying "He's going to make the bank wait."
Nevada to me, in the US, the most common meaning for "hold up" is to make __ wait or take longer.
Hold up the line Hold up the mail Hold up the traffic