#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]

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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.