Go ahead and take the sign back. You can get away with most things as long as you don't physically harm someone. Think of it this way. If the guy can't afford to pay for his sign then he can't pay a lawyer to go after you for taking it back. I also like the idea of replacing it with a sign repossessed due to non payment sign.
If someone steals your car, you don't need a lawyer to go catch the thieves.
Take it down a notch...... you go to the store and fill a cart full of groceries. Instead of paying, you tell them you'll pay them later and run out the store. They don't need a lawyer to run after you.
Where are you people getting your logic ??
Just because you have the ability to take it down, doesn't mean you have the right to do so.
It's very simple.
You did the job and whether or not it was done correctly or misspelled or wrong colors, that sign belongs to whoever's wall you put it on. They're inability to pay you is on you and that's a civil matter. Needs to be taken up in court, with letters, lawyers or whatever path you choose. It's not their fault, you didn't follow through on
YOUR OWN contract and didn't collect the money, regardless of what the scumbag promised you. Now, taking the sign down with or without permission is really stealing. It's totally besides the point that he didn't pay you, but you're still stealing someone else's UNpaid for merchandise. It's still stealing. That is a criminal offense. You have now entered yourself into a different form of law, from civil to criminal and depending on what the sign is worth, you could even do jail time.
You people telling someone else it's alright or just do it should be ashamed of yourselves. No matter how easy it is....... you can't tell someone publicly or behind the scenes to do something so stoopid. Keep that kinda advice to yourself. Suppose someone followed your advice and got caught.... are you gonna bail them out and pay attorney fees and all the other stuff for your dumb stoopid wisdom ??