Just my feeling on this: if the board guidelines--quite rightly--do not allow any post which disparages the royal family in any way then surely just deleting the offending post solves any possible problems. Why go to the extreme and ban a Member when the offence itself can be wiped out almost immediately?
Was there any fore-warning to the Member?
Banning seems very harsh for a one-time offence. (At least I assume it's one-time)
