This is a very valid point.
In theory, it's relatively easy to physically stop the small boats at sea. Heck, we could even elect not to pick any of the boat people up, accepting whatever consequence that would mean for those on board.
If we did this, and stuck with it, it's safe to assume the small boats would stop within a relatively quick timeframe.
However, let's say the government decided to do this, whether it was or wasn't deemed legal. Let's say they decided to bash on, accepting whatever the legal consequence would be from UK and international quarters.
I suspect this is where it probably wouldn't happen, because those tasked with stopping the boats and not helping those on board would most likely, and understandably, say 'nah, not doing that.'
I like to think, if government ever proposed anything that crossed a line into unacceptability from the viewpoint of most Brits, that the masses would ensure it didn't happen. This links in with us being a democratic country.
I for one want the small boat crisis stopped. However I would never support a strategy of literally stopping the boats and pushing them back, with no regard for the safety of those on board,