I can think of several solutions:
1. Ignore Newbies when sorting on "Last post" date. This way, Newbies can't bump threads anymore.
Jr Members should be added to this also, as the requirement is only activity, they will just create a thousand more accounts, post garbage to level up and add more accounts to their army of bumpers.
I know it might sounds harsh,but it does not affect the user at all, they still get to share their "opinion"

2. Nuke all Newbies who have deleted more than x (say x=2, make it x=5 if they have more than 14 posts) of their own posts.
3. Take away the possibilities for Newbies to delete their posts.
I think this should be enforced for everybody on this forum.
Lots of hacked accounts are deleting their history, shills are deleting proofs after the
scam ICO has finished and many more....
I'm active on another forum where you can't edit your posts after 1 hour nor can you delete them, and it's pretty funny especially in the politics section to look at the users history and then rub it in their face

.
4. Require a captcha for Newbies for either posting and/or deleting a post.
I would love a captcha that would make them read the rules on the forum, like typing the second word, the last character or the first of a random rule around here.
But...whatever it is, something must be done!