A colleague at my new job keeps insisting that we go out for coffee and lunch. They are obviously a polite person, but I’m afraid that saying yes to this will disrupt my balance at work. I don’t want to become someone who mixes work and personal life. Would it be professional to say no, what would you do?
I experienced the same thing. In my first job, a coworker kept suggesting we meet outside every week. I went once at first, and then it became difficult for both work and personal life. They think you’re always spending time together. So, I believe it’s better to be clear from the beginning.
There’s something like this; when you say yes in this situation, it can create an impression like “we’re close friends outside of work.” I told someone no at work, and the matter was closed. So, it didn’t make anyone feel stupid. Besides, if they’re a decent person, they’ll understand, I think. 
I think you can say no in a simple and polite way. You could say something like, “I’m currently taking my time outside of work for myself.” If it keeps coming up, it’s time to maintain some distance for this work 
But this is confusing: You’ve often said that he’s persistent, which seems to bother you. So does this coffee or dinner invitation make him closer to you in a work sense? Maybe he just wants something purely social?
In such situations, it’s usually important to pay attention to hierarchy and power dynamics in the workplace. If this person is in a higher position and is questioning your right to say no, that could be a bad signal. If not, simply asking for some distance is enough. Even outright declining can be professional.
Actually, I’m starting to feel a bit more certain than I was thinking just now, but one thing I’m struggling with is: being both a nice person and someone fun within the group. I’m just worried this will turn into an individual connection. That’s why it’s complicated 