Well, first of all - why waste a perfectly good drink. Accepting the drink does not obligate you to anything.

Personally I like it when men buy my beer or get me some wine (if they see that is what I am drinking.)

I feel like making new friends is good, healthy and fun. Just be clear if you are not available or not interested. Boundaries!!

I live in bars and I haven't see anyone do that in decades. i'm sure it happens but rarely. I think it was more of a movie thing.

Just decline most will understand once they know how you feel

Nobody , and I mean NOBODY , will ever be allowed to do this to me . Not on my 20s and not now , a year away from 50s. They have and I send it back . I am nobody’s paid monkey .
If anyone wishes to get me to talk to while sitting on a bar ( I normally eat at bar sections bcz one person , except if my dog w me ), they can do so without assuming that I can drive w another drink .

Some guys will send drinks; others will just come over and start a conversation. If you want to meet someone, it's better to sit at the bar. Bring a female friend so you don't stand out as a target, though. As far as declining, just say "No thanks, I'm taken", or simply "No, thank you." Make sure you can make a safe exit if someone is harassing you (find a bouncer to escort you to your car), and NEVER leave your drink unattended.

I haven't done it in about 40 years and I guess it depends if the bartender asks you first or just brings over a drink. Either way you can just say, "no, thank you." But if you look over and see who it's from and don't like him it's going to hurt no matter how you do it.