Look at him/her in the eye and talk as clearly as you can.,
Go straight to the point and admit that you are just not interested.,
Don’t go back and forth.
Even if it is loud around you, even if you feel like he/she might not be as sober as you are, just make sure that your message will be said and understood. Once in for all.
, You are not making fun of anyone here, just being honest and as straight forward as possible. Forget about this “it’s not you, it’s me” crap. No one likes a liar, and everybody hates fake people. However, only refer to facts and try not to attack him/her with emotions. Don’t say that his/her conversation is boring. On the contrary, say that it is interesting, but that it is just not something you want to talk about.
, Don’t say all these things and then apologize (because, remember, it is not your fault). Admit that you like the attention but make it clear that you don’t want to go further and wish he/she let you alone. Try to make it sound definitive without creating a cruel “doomsday” atmosphere.