Can't you just send a written letter to Lib Dem HQ?
I'm not sure about tweeting the account itself. Never wrestle with a pig. You both get dirty and the pig enjoys it.
You could compose a printed letter with screenshots of a couple of the more unhinged stuff and ask them to look into it. Be very factual and cool. Escalate into an official complaint if they try to fob you off.
You won't be the only Lib Dem member to be thinking this way. That account is reflecting very poorly on the Lib Dems.