You can write a script using the API to delete your comments.

If your a instance admin you can nuke a user and all their content in a few clicks

If the information is very sensitive then you need to do more then mark messages as deleted, federation means copies are everywhere. The best you can hope for is update the message to something else, wait a few days for it to federate, then delete it, wait a few days for that to federate, then nuke it