As I see it, the options are as follows:
simple bans:
Ban Liepard (+purrloin)
Ban Assist
complex bans:
Ban Assist+Prankster
Ban Assist+Phazing
(or leave it as it is, but i'm 99% certain that everyone agrees that something should be done)
Since there is not much difference between the Assist simple ban and the Assist+Phazing complex ban (since assist is rarely ever used besides this strategy (except perhaps divecats, which is much, much, MUCH more situational than this), I'll treat them as one and the same. We are not restricting teambuilding at all by banning a move that is not broken at all on most mons, because anyone with an IQ of a chimpansee wouldn't even think of running Assist (altho the average PS user seems to come quite close to that, no offense)
Why we should not ban Liepard:
Liepard has 3 common sets in NU:
1: Substitute/Swagger/Thunder Wave/Foul Play @ Leftovers
2: Pursuit/Sucker Punch/Encore/U-turn @ Dread Plate (or a variation of this)
3: Assist/Substitute/Nasty Plot/Dark Pulse
Set 1 isn't broken, it's merely annoying. Even if some of you argue this is broken, i'm sorry to say that there are a lot more pokemon who can pull off this strategy (e.g. Murkrow, who even has roost) It's obviously not something people enjoy facing, but that doesn't mean it's instantly broken. By using creative sets (for example Lum Rock Blast Golem), as someone pointed out, this strategy can be 100% beaten.
Set 2 is the sole 'positive' set Liepard has. It's a great addition to the metagame, fitting amazingly on momentum-based teams, teams that have trouble with boosting sweepers or teams that simply need it's utility. I'll make this clear: I believe we should NOT ban Liepard solely because of this set's existance.
Set 3 is broken. It has, what we are calling 'counters'. But in reality, i'd say these are the things that come closest to countering it. Even packing 1 or 2 of these counters and you might lose, as illustrated by some of the replays posted in this thread.
So, in my opinion, we should not ban Liepard itself. Then, the choice lies between two sides:
1. (Ban Assist/Ban assist+Phazing)
2. Ban Assist+Prankster
The difference between these two lies in non-prankster assist abusers, most noteably Persian, Sneasel and Purugly
If these are deemed broken, choose option one.
If these are not broken, we should ask ourselves this question:
If a strategy is not broken, but will still allow a less skillful player to beat a top-ranked player mindlessly, do we really want to keep it? Persian might not work in X% of situations, but that means it still works (100-X)% of the time. On the ladder, this might lead to an average result. But in tournaments, this might become a viable strategy. Do we really want this? I believe that is where the question lies at this moment.