I'm not sure that Stella really is any different from any other 5% lagers, or if it's drank by a particularly volatile type of customer, but certainly a number of pubs in Sussex are no longer stocking it because of its reputation.
As much as I think it is a crap beer I refuse to believe there is any special chemical within which accelerates violence. I'm sure a gallon of Kronenbourg 1664 would have the same effect. So while I'm pleased to see beers such as this being removed from bars, I am sure the replacements will be other dodgy 'European' lagers brewed in the UK.
If anybody has any anecdotes about the Stella effect then please share them, but my abiding memory of being with someone who had downed a whole load of the stuff certainly didn't end in violence, and as that imbiber is female I won't go into detail.