Give him a free sandwich, a handshake, and wish him good luck job hunting. If that doesn't work, tell him he's banned from the restaurant and if he sets foot on the premises you will have him arrested.

I'm guessing you told him why he was fired? He need to be aware of his incompetence, otherwise he will believe that he was unfairly treated. If he knows that he is not suited for the job, and is mentally stable, this shouldn't be an issue.

You fired him, you now have nothing to do with the incompetent fool.

Can I get a free shwarma if I visit your establishement? It's effing delicious. Often make it at home.

Anyways, that's besides the point, you've had the incompetent fool fired, first off if he comes back, tell him he was incompetent and that's why he needed to gtfo, but if he keeps coming back and harassing your restaurant, your employees or your customers, call the cops and having him removed and barred from your restaurant.

I'm not sure what country you are in but here in america we call this a restraining order. if you can get one, when he trespasses the idiot will be spending a little time in jail.

Lots of good advice.

Firstly, make sure you document poor performance at the workplace. Have a conversation with the employee to discover if there are any mitigating circumstances why their performance isn't up to par, and then define a series of action items the employee and or you will do to ensure success. Document (take note) of everything he/she says, than file it. Do this at least three times.

The exception to this rule is if the issue isn't a performance issue but detrimental to the business. For example, pissing in the coffee, stealing, etc. Things that would provide immediate cause for dismissal.

If you define the performance problem, identify any performance obstacles, and action items the employee and/or you need to take to succeed, and he/she still doesn't improve after those steps have been taken, than you have a documented cause for dismissal.

2. As someone else mentioned, you have a right to conduct business without disruption of that business by anyone except the authorities with cause. Therefore, direct him to leave the premises/property.

3. Contact the authorities if he/she continues to be an issue. If necessary and possible in your location, obtain a protection order if he/she continues to defy the local law enforcement.

4. DO NOT engage in personal attacks. NEVER EVER! If the person chooses to discuss the issue in public, reiterate the fact that you worked with them 3 times over performance issues and they repeatedly failed to improve or follow through. In this specific case, there wasn't a task you asked him to perform that he performed up to your reasonable expectations.

5. Likewise, DO NOT engage in retribution. If an employee is disruptive to your business, use local law enforcement to your benefit. Let them be the heavies. Never make a "special sandwich" or engage in any other type of retribution. Not only could you harm your reputation, but you could kill your business.

6. There was one other item I thought of but I forgot. Good Luck!

EDIT:  Remembered!

Why would this guy want his job back.  He's telling everyone how bad of a business it is.  So it's only a bad company if he isn't employed there?  How does he, in no position of authority and unable to properly do his job, make it a better company? 

call the cops.

Other than that, before i ever fire any body, i sit them down, I talk to them about exactly what they are doing wrong. I ask them if they want their job or what is wrong, if they say they don't want the job it ends there and they are fired/quit. If they do want the job I tell them they need to step up their game, and list what they need to do to keep their job. I tell them they have 1 - 2 weeks to prove themselves. and after that they re either a good employee or they are fired. I work at a pizzeria so we probably have similar typed employees.

Ultimately deluded religious fanatics (there are good and bad "fanatics" in my view) best respond to the immovable reality of the authorities.

After following Adinnieken's great advice, he will learn quickly when cuffed that he is not as righteous as he might think.

let the fists speak

plenty of workers get fired or sacked from jobs all the time. only a minority stalk/harass a former employer. if genuine about getting back at employer the former employee would make an unfair dismissal case.
a better recruitment screening process of new applicants in the future may be needed. insist on 2 or 3 years experience in sandwich making and 2 or 3 contactable references. that would sort out who is genuine about working.