I'm quite positive the driver in question was NOT bumpdrafting - we were racing on a SO config with XRGs, the speed difference so great that he catched up on me halfway through the back straight.
Instead of attempting a proper pass, he started bumping me - I blinked my brake lights to no avail, and eventually brake and let him pass, I did this to avoid a yellow flag in case he would try bump'n'steal-ing on the coming corner.
A couple races earlier I was spun in a similar situation (same spot, different driver) and being on Race 1, even if I drive cleanly my yellow flag ratio keeps increasing because someone will either crash into my car or in front of it.
IMO this is just an unfair way to make an easy pass on a slower driver, as such I would expect it sanctioned by the rules but it appears to be not, hence the question