Ex:(

Tell him no, you don't want to go out with him. Then tell him you've moved on, and you think it's time for him to do the same. I would tell him in a public place or at least be in your home so that if he turns violent or gets really angry, you're not alone to deal with him. Just because he clearly hasn't gotten the hint yet.
Delete his contacts and delete him from social media. Make it harder for him to contact you.
Good luck dear <3

I think your best bet would me to sit them down and tell it to them straight and bluntly, that you are completely and 100% not interested. repeat it if you have to. just make sure they understand you. otherwise they'll keep wistfully chasing you around, thinking he has a chance. that would be the easiest and quickest way.

Ask to have a talk with him. Tell him how you feel and be completely honest. It is better to let them know the cold hard straight up truth rather than leading the person on. Don't keep his hopes up.

Try not to encourage him - if he starts flirting be really blunt and straight faced. If he asks you out say no and don't continue a conversation. You don't need to hurt him, just be honest. it doesn't work between you two.