From a practical standpoint they absolutely cannot do it - the border would be impossible to maintain in a 'hard' way - there essentially is no border between NI and ROI as anybody who has been here will know so to try and set-up checkpoints, etc. would be virtually impossible to do - and no I do not believe there is a technical solution to it. If they ripped up the Good Friday Agreement it would be political suicide and would cause civil unrest - not only that it would be illegal as I've already said.
Believe me - a hard border is an impossiblity even if that is what the UK government wanted to do (which I do not think they do). They will fudge it and continue with some sort of customs union.
What are you arguing about? They can violate it, the only question is whether they are willing to pay the financial and political price, that's the doubtful part.