I don't know how that is done in a democracy - where the red lines are set - by whom and how.
Red lines are set by consensus of opinion.
If consensus of opinion shift, what is deemed acceptable shifts.
Farage was not getting air time previously because his views were not thought to represent opinions that the consensus thought was acceptable.
As consensus has shifted to the right those views, whilst unpalatable to many are now acceptable to others. Hence he gets airtime.
Banning him therefore would do nothing. It would make those who felt he had a point feel like they were being disenfranchised in a way.
The horse has already bolted on the fact that he is now viewed as part of mainstream thought, even though this is still on the fringes and he isn't a front bencher. Remember he IS an MEP so does have a certain status with regard to the referendum in that respect and is felt to have brought about the shift in consensus by many that has led to the referendum.
The only way to get rid of his air time if to make him irrelevant again. This is either by proving his argument is not accurate, or otherwise shifting consensus back to the centre again in someway.