Yes the practice of boosting an account (either by grouping up or by accessing an account that belongs to another person) is bannable, but the behavior itself really doesn’t need to be reported unless there is some confirming evidence, such as the suspected user admits to doing so in match text chat. I asked Jeff Kaplan once about how we should go about reporting boosting accounts and here is what he said:
If you find that you are badly defeated by a player (of any XP level/portrait border) but they don’t say anything to suggest they are engaging in account manipulation, then you should not report that player. Overwatch is notorious for having “snowball momentum” and that can put emphasis on certain players who can look “god-tier” when they may just be having a darn good game.