If you’re not keen on someone it’s common to react negatively to them chatting you up. Sometimes this leads you to overreact and feel like a bastard. However, if you act clearly and assertively, you won’t feel so bad.
It’s particularly important to not give any mixed messages since they’ll hear ‘might be interested’ if you’re not specific. Saying things like ‘I’m flattered you like me but’ may mean they only hear ‘you like me’.
You can try things like ‘I’m afraid I’m not for you’ or ‘I’m already in a relationship’ or ‘I’m not interested in a relationship/ sex with you’.
You need to gauge the situation by the person concerned. Someone who is drunk or feels threatening may need letting down gently for your own safety. But generally it’s better to be clear, firm and friendly. Of course they won’t be happy but it’s better to be honest at the outset rather than wasting their time and yours.