I havn't had this problem yet on my GT, it was on a fiat that has similar suspension.
In my case the was a collar either side of the ARBs and one had dissintergrated. The whole anti roll bar moved accross and my drop link fouled my suspension.
I did a quick fix by centralising it and putting a small U type clamp (16mm to 20mm I think) on it to stop it moving accross again.
I did replace the whole anti roll bar in the end as the bushes were worn, and bushes weren't widely availiable seperately at the time.
The new fiat anti roll bar had collars on it.
I don't think anti roll bars should have any lateral movement.
In an ideal world maybe unworn bushes would hold it central on their own,
Maybe see what the Alfa dealers parts dept have to say
Hope this helps your fix