The simple answer is, not a lot on your own.
You could talk to other committee members and ask them to put some pressure on this person or you could wait until your next general meeting and propose a motion that this person be removed or not re-elected to the committee.
I’m not sure when the new laws come in, but we will soon be able to remove committee members from the committee by a simple majority at a general meeting.
You could propose than motion and ask it to be added to the agenda of the next AGM or EGM. You will be allowed to attach an explanation of why they should be removed. Stick to the facts – filter out emotions and opinions – and at the very least their behaviour will then be a matter of strata record.
And when they threaten defamation action – as they will – point them in the direction of this news story from a couple of years back.