sage words pfr - - however after several of us had tried to engage in conversation with said observer (who really did not 'get' what the discussion was about)and some people were becoming more and more frustrated nay enraged (I include myself!) the realisation came that the best way was to completely ignore this person, carry on with discussion as if he (for it is a he) was not there at all!
Which is fine - - 'cept there were particular issues to discuss and some of us felt uncomfortable sharing these with a person whose sole purpose was to disprove and disrupt.
I am certain that this troll would not open his mouth if he was in the same room as us for fear of showing his limitations and ignorance -- he is hiding behind his anonymity.
None of us will succumb to the temptation to respond to him in future!!