A smart plagiarist can escape ban if they delete the evidences before a moderator handles the case.
That's why we always archived evidence before reporting, so the mod doesn't have to go through all of your posts. Most mods base their decisions on archived evidence, so deleting your posts won't help you much; it will only prove that the user plagiarized on purpose.
What you said is what I believed, that deleting a plagiarised content before a moderator sees it will mean you plagiarised on purpose and will lead to the person being banned.
But some cases proved me wrong. Where a post will be reported in the plagiarism thread, before a moderator will treat it, the plagiarist must have deleted it and go free.
I also remembered that same case raised a topic of discussion when a staff member - Xal0lex unofficially said that moderators most times if not all the times does not use evidences provided by third parties like ninjastic etc, except the ones available on the forum.