Especially when a woman refuses a man, because sometimes the other person may not understand you, because he thinks you will give him another chance, so he will spend some time chasing you or try various other methods to chase you.
In fact, if you really feel inappropriate, it is also a waste of time and energy paid by the other party. It's better to be clear when you speak, and try to be tactful. Just make it clear why they are not suitable, and don't blame or hurt each other's self-esteem. Don't say what's wrong with each other, just that we are not suitable in some ways. As long as you have a firm attitude, I believe that the other party will soon understand what you want to convey.
You can even tell a white lie, such as telling him that I already have someone I like, or that I already have someone I am dating, so I won't have any further relationship with you at this stage, or telling the other person my future plans, such as going abroad, or what career plans I have, so I don't want to start this new relationship.
Many girls like to say that we are still suitable to be friends. I really don't feel that way about you, which is not impossible. But once it comes to being friends, you may have to keep a certain distance from this boy in a short time, otherwise the other party will still feel that there is a chance. In other words, after you say this sentence, when you encounter some difficulties or feel depressed again, don't always ask for help or talk to each other.