All of the suggestions to “prevent” this sort of stuff is too narrowminded. You’re only trying to deal directly with the problem, without looking at the repercussions.
However, there are ways they can discourage boosting. Although, as Broxiz said, account-wide titles and mounts leads to people easily confusing alts with mains.
That doesn’t mean there aren’t people boosting or coaching, but its real number is probably less than people are saying.
So the way to discourage boosting would be to make it harder. It can never be prevented completely, but the system can make it a lot more convoluted so that it doesn’t become as popular because of the increased amount of effort it’d require. (For example, how there aren’t as many smurf accounts in WoW as a subscription service compared to moba games where accounts are usually free to play. If WoW would be f2p it’d undoubtedly lead to more smurf accounts, for sh!ts and giggles. But it’s not as much of a thing because it’s discouraged with the cost to play, so that’d be the same effect it’d have on boosting if boosting would be made more strenuous.)
An example of making it harder would be:
Depending on how long those “penalty games” would last, it’d make boosting someone new take that much longer each time. Make it long enough and fewer people would even be bothered to pay for it to begin with, since they’d have to wait until they actually get their money’s worth. (The thread was about the lack of social glue though, but it can also have that kind of effect on the side.)
The rating system has more problems than just that though, the gearing system is messing things up royally. It’s making rating climbs require more games than it should, and for some comps it’s affecting them much more than others.
In other words, ilvl has a very REAL impact on win potential, and with these insane ilvl differences and so many different “walls” in place with upgrades to ilvl rewards, together with the time gating, it’s inherently creating large ilvl gaps between different kinds of players.
It isn’t a concern once everyone is equally geared up, but it’s designed so that the majority will never be equal in ilvl in realistic conditions. (It’s not impossible, it’s just very unlikely.)