You've learnt an expensive lesson!
New Customers should pay a deposit.... too late for that one now.
Did you receive an Official Order (if not you're up the creek) if so, was it in his name or his business name - if it was him personally you can sue him personally - if it was his business entity you're basically up the creek if goes bankrupt.
Technically he owns the
signs and he owes you the money. These are 2 separte issues in the eyes of a receiver if he goes bankrupt, so removing them or damaging them 'could' land you in trouble - that's not fair but it's the (English) Law.
In future (too late on this one) you can protect yourself to some degree by 1) getting an Official Order 2) Having him
sign (as a new customer) a personal guarantee 'wrapped' in the wording of a 'credit' application for a 30? day account 3) have what's called a 'Romalpa' clause on your Invoices (Google it) - this is a clause which states that title to the goods (
signs etc) does not pass to the buyer until fully paid for. This was a famous case in English Law which was upheld in favour of the seller and is used extensively these days (you' may have seen it on the bottom of your supplier Invoices but thought nothing of it) This would allow you to remove any
sign which, on balance, was economical to do so.
BUT - revenge is sweet, I think I'd be covering the window prints with some cheap unprinted vinyl to minimise his advertising - sounds like he never intended to pay you.
Kendo