My first line of defence is not even acknowledging their existence. Usually this works. It's hard for them to keep engaging when you don't provide any content for them to work with.

Next step if they just won't stop, is a polite refusal if I'm really uncomfortable and don't want to / can't move away.

Next step is calling it like it is in a matter of fact way. Such as asking them if they think bothering me is actually impressive or effective in any way. And being very verbally clear that I do not want their attention. Simply put and direct, like I'm talking to a child. Then I go back to stonewalling them, all the while looking for an opportunity to remove myself from their presence.

It's never gotten this far but next I would threaten to call the cops. Because if someone is following me after they are aware that I don't want to interact with them, then I am not safe.