I have started threads for roleplays over the years and for various reasons, the roleplay was cancelled before my partner ever made the second post. Am I permitted to remove these starter only post threads or delete the contents of the one post?
If there are no other posts from anyone in it but you, you may delete the post since it is not affecting anyone else.
Generally the deletion rule is something we come down on when the deletions affect other people--when it removes other people's RP posts, when it destroys the continuity of an ongoing RP, or messes up the integrity of a discussion when someone removes a reply that influenced the way the conversation went (in other words, abuses of the deletion privilege)--just to name a few things (among several things) Staff takes into account about deletions & deletion requests.
And of course, if you aren't sure if you should delete something, the safest bet is to PM a Staff member with a link to the post and ask if it's alright to delete it.