Which is very easy to say, and a damn sight harder to do - have you had a go?
I'm not gate staff, but I have refused boarding to several who have got past the gate staff and have arrived at the door of the aeroplane.
Why dump the responsibility on the gate agent
Because they are the first "official" person that these chimps have come across since they got themselves through security and into the duty free shop and airport bars.
Milhouse999 - as this is the Jet2 thread I will confidently state that those 2 would have been dealt with very differently, even if a delay was incurred - ZERO tolerance.