If you want to delete your posts then yes you can but remember you can't delete a thread which is even created by you so you can only lock and unlock the thread and only mods can delete the thread.

Anyway I wonder why you should care about your rank when you want to wipe your forum profile?

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the ranking criteria is simple you need certain activity as well as merit to reach certain rank so if you fail to meet one even not let you there and FYI deleting a post will also create a loss of activity created but in most cases it won't affect the rank because generally number of posts will be higher than number of activity.