It is interesting that something like bullying and intimidation - so much in the news these days - is such a subjective issue. For example, I find the language used in the previous response, with bold text and different sized text in sections discrediting the opinions of the original post is more than just an expression of personal views but is intimidating and bullying. I doubt the author would agree.
To go back to the original question, you cannot stop anyone having a conversation about anything so if people who happen to be members of the burial group want to have a conversation about a staff member down the pub, as unwise and lacking awareness of staff management as it might be, it is difficult to stop that happening. Is it a formal meeting which could be used as a factor in a staff disciplinary issue? Probably not and I'd suggest it might provide fuel for an action to an employment tribunal by the staff member. Your council should have a discipline and grievance policy and if a complaint is raised by anyone against any staff member, that policy needs to be followed. If you haven't got such a policy, there are lots of good examples online and as I've said before, taking advice from an HR expert or ACAS is advisable. This protects both the council from malicious grievances by staff and protects staff from malicious acts by councillors and others.