Well stated
No legal advice here (for the reasons stated in the above post), but as an overall course of action, I think I would -
1) get a lawyer
2) Have the lawyer obatined in #1 go over the contract to make sure that it's as ironclad as you say, to make sure that you're in a good position for #4 in case #3 doesn't work.
3) If you have a strong position per #1, have the lawyer send a letter or two to the customer outlining your position and what you intend to do about it ( this would be #4). Showing that you are serious and won't go away might just get them off the dime and pony up.
4) If #3 fails, pursue you claim, with or without the lawyer, as your state allows and by whatever means that your state allows (the method of this recourse is indeterminate as you didn't supply the state this is in).
As the preceeding post states, without you stating the State, the above statements, as stated, are a very general course of action to attempt to collect the debt.
Hope something in here helps,
wtd