not providing any incentive to create further bogus accounts.
I disagree. I've never heard of this phenomenon happening beyond a few very unusual cases. Trolls, in general, aren't after XP. In fact, many are happy to see their XP sink to plutonic lows. Some appear to have taken pride in the rate of their XP decline.
Thus, if anything, forcing them to abandon their accounts after reaching some negative XP threshold might be more of a disincentive.
A word spoken in Mind will reach its own level, in the objective world, by its own weight