Tenants let dogs go bathroom on deck- advice.

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I feel I should preface this by saying that the tenants were ones I inherited for only two months.  

The first time I went to a showing on the property I noticed that there were appox. 10-15 piles of dogs poop on the 2nd floor deck.  We are talking a standard deck off the back of an up-down duplex.  When we took over possession of the property we immediately sent a warning to the tenants that they need to remove the dogs poop.  The next couple times I went over to the unit there was no poop on the deck.

Fast forward a month and the tenants moved out.   My wife and I went over to meet them and they had just left keys and opted out of the walk through on the property.  They left the property in very good shape which I was obviously happy about.  When we went out on the deck, we were hit with a wall of waste smell.  Mainly the smell of pee/ammonia but also residual poop smell.  It was so bad that it was actually a little gross to stand on the deck. 

The move out was only a couple days ago and I still have their deposit.  I am not quite sure what/if any action I should take as far as their deposit goes.  I have to have the deck treated/cleaned in some fashion.  I just can't possibly move new tenants in to that unit with the deck smelling that way.

I was wondering if anyone else had come across this and what they did?  How they got rid of the smell?  Am I in the right to charge them to have this cleaned?

Thanks in advance!!