Hello frozenpizza,
I am much older than you, so allow me, to give you my opinion.
I think, you can be really proud of yourself, because you listening to your gutfeeling. If you feel uncomfortable, just stop communicate with this people.
Writing with penpals should be fun, not stressing you out. Or creeping you.
If you want to polite just tell them: Sorry, I cannot write with you anymore. - But you do not have to do this! You don't own anyone an explanation.
If the people still writing to you, just delete their posts without reading.
Everybody, but especially young people, have the right to decide, who you want to befriended with. You have the right to change your mind, even if you startet the correspondence.
It is so great, that you are able to talk about this.
Disclaimer: Old-woman-advice 😃 - Many brains are wiser, than one alone. You did good!