However, I may briefly reply to emails where it is evident the guy spent a lot of time and effort writing it, or if they specifically ask me to tell them whether I am interested or not. Someone asks you out on a first date (outside of online dating) Reply with something polite and simple. Examples: I am flattered, but unfortunately, I do not feel the same way.
Unless you had a mini- or long-term relationship, there is no need to give them a reason why you are not interested.
It is in your best interest to be graceful when breaking up because the other person may impact your life somehow, either in the present (e.g., you have mutual friends and need to get along) or years later (e.g., they become your boss).
If he does ask you out again, then it is courteous to gently tell him no.
Examples: I had a good time but I do not see this going further romantically.
If you are concerned that they have deep feelings for you and will be heartbroken, that is even more reason to end things earlier.