You aren't allowed to discuss cheating in the forum and especially not to post things identifying individuals.
There is a club called the Cheating Forum for such things.
But in general, yes, sandbagging is against site rules. (Manipulating ratings.) You should report such things. (Use the report button on player profiles.)
I've come across a pretty strong player rated Bullet etc above 1700 in a Daily tournament I'm playing. However, his Daily rating is always below 1200 because of lots and lots of this before tournaments...
[image removed -- VP]
Is this actually cheating - or just considered bad sportsmanship?