#Taxi, 779 bytes

Go to Post Office:w 1 l 1 r 1 l.Pickup a passenger going to Chop Suey.Go to Chop Suey:n 1 r 1 l 4 r 1 l.[c]Switch to plan "e" if no one is waiting.Pickup a passenger going to Charboil Grill.Go to Charboil Grill:n 1 l 3 l 3 l.Pickup a passenger going to The Underground.Go to Writer's Depot:w 1 r.[p]; is waiting at Writer's Depot.Pickup a passenger going to Post Office.Go to Post Office:n 1 r 2 r 1 l.Go to The Underground:n 1 r 1 l.Switch to plan "n" if no one is waiting.Pickup a passenger going to The Underground.Go to Zoom Zoom:n 3 l 2 r.Go to Writer's Depot:w.Switch to plan "p".[n]# is waiting at Writer's Depot.Go to Writer's Depot:n 3 l 2 l.Pickup a passenger going to Post Office.Go to Post Office:n 1 r 2 r 1 l.Go to Chop Suey:n 1 r 1 l 4 r 1 l.Switch to plan "c".[e] 

Try it online!

Ungolfed:

Go to Post Office: west 1st left 1st right 1st left. Pickup a passenger going to Chop Suey. Go to Chop Suey: north 1st right 1st left 4th right 1st left. [c] Switch to plan "e" if no one is waiting. Pickup a passenger going to Charboil Grill. Go to Charboil Grill: north 1st left 3rd left 3rd left. Pickup a passenger going to The Underground. Go to Writer's Depot: west 1st right. [p] ; is waiting at Writer's Depot. Pickup a passenger going to Post Office. Go to Post Office: north 1st right 2nd right 1st left. Go to The Underground: north 1st right 1st left. Switch to plan "n" if no one is waiting. Pickup a passenger going to The Underground. Go to Zoom Zoom: north 3rd left 2nd right. Go to Writer's Depot: west. Switch to plan "p". [n] # is waiting at Writer's Depot. Go to Writer's Depot: north 3rd left 2nd left. Pickup a passenger going to Post Office. Go to Post Office: north 1st right 2nd right 1st left. Go to Chop Suey: north 1st right 1st left 4th right 1st left. Switch to plan "c". [e] 

Explanation:

Pick up stdin and split it into characters. Covert each character to ASCII. Print ";" as you count down from that ASCII to zero. Print "#". Pickup the next character and repeat until done.