If you're no longer using it, I assume that within RP conventions you can just say you're done with it by editing the original post, as usually there's no reason to delete a RP post because you're simply done with it. I'm not a roleplayer, so there might be something unique convention there. I just don't see a point in deleting purely for that reason.
If your post was the last post in the thread, you can delete by pressing the X next to the edit button in the top right corner of the post. Otherwise (and if you really need to delete it), you can edit your post and just replace it something like "deleted".
Edit: Looking at the forums, it appears that adding [CLOSED] to the title means that only select players can roleplay in the thread, and old unused threads just get ditched and nothing else happens.
Last edited by Bormiar on Fri Jun 05, 2020 11:28 pm, edited 1 time in total.