I believe it actually is. The Report Harassment option in-game includes harassment via gameplay means, and this would fall squarely under it. But as brought up before, it's a very gray area, because how can you prove they were doing it intentionally instead of just, well, being bad?
Granted, if you have the proof, go for it. It's very much a reportable offense.
Blist sadly won't stop people from getting queued with that person. It's a glorified chat filter.
