I usually chat with my online friends with the simple intention of sharing my daily life and making friends, but the other person obviously wants to move towards a relationship.
I've made it clear that I just want to stay friends, but he still sends me flirty words from time to time.

The situation left me torn and unsure if it was appropriate to continue the conversation.
Worry more about whether you'll look scummy this way.
Hi, I can empathize with your confusion and helplessness right now! You're doing the very right thing, if you don't have feelings for the other person, making it clear straight away that you're only friends and avoiding blurring the lines is key. But why is he still sending flirty messages? Possible reasons are: one, he hasn't given up yet, and thinks he can change your mind by insisting, so he uses ambiguity to test the waters; two, some people don't have a strong sense of boundaries, and it's easy to confuse the relationship between friends and lovers; and three, it could also be intentional harassment. In any case, if you don't like it, reject it clearly again and tell him you don't accept this kind of information. If he still does not change, decisively delete contact information. Remember, a clear attitude is the only way to protect yourself from dangerous situations.