On the post-game score screen, you can use the drop-down on their name to report them. If you missed your chance after the game, you can access this screen later via your Match History tab.
Open the drop-down menu, select the "More" option, and then pick "Report Player":
A new window will pop up where you can select a reason (in this case, "Cheating" is probably most appropriate) and then enter an explanation.
I don't know if it is technically violating the rules. The lead developers have commented in multiple interviews in the past that they don't like players smurfing, so at best it seems to tiptoe the line. In this case, the player you're talking about is likely losing a bunch of games to be placed in a lower league, so they can rack up a bunch of easy wins to grind out the "Win 1000 matches as X" achievements.
I am not sure if you can be matched with a blocked player. (In Heroes of the Storm, which uses the same engine and is made by the same dev team, you can, but it's also a modified matchmaker.) But, by reporting the player, you will bring them to Blizzard's attention and they can investigate and respond appropriately.