Hmm. Before I ref any game I can't be sure that any of the players won't stamp on the head of an opponent. How can I ref that safely (an over exaggeration to make a point)
You appear to be saying because one idiot has tripped a player the a repeat is more likely. I don't follow that logic.
The logic is simple. You start the game assuming that everyone is going to follow the rules; that each side will be disciplined, and will follow the rules of the game. Any foul play by a player is dealt with within the rules, and in particular any player who commits serious foul play will not have the chance to repeat the offence because he will be remvoed from the field of play. Even so, if you feel that you have lost control of the game so as to threaten the physical safety of the players, I would have thought that you have no option to abandon.
We are however dealing here with spectators. You have no disciplinary control over spectators; that is down to the club. If the spectators get out of hand, your only option is to abandon. You already know that the club is unable to control at least one spectator, to the extent not just of voluble dissent or abuse, but direct intervention in the course of the match within the pitch; without investigation you won't know whether there are any others who may repeat the offence. If the club say that the parent involved is a known malcontent, then their judgment in permitting him to attend is called into question such that you can't rely on their assurance that there will be no repetition; if they say they didn't expect it, then once again you have to question the reliabliity of any assurances they may give, because they clearly don't know who may be capable of such an act.
The reaction post event is key.
And the reaction may well persaude you that a subsequent match can safely be played there. But you don't have the luxury of reviewing the club's reaction post-event, because you have to make the decision on whether to continue with the match before recommencing.
I realise that you refer to the reaction by way of exclusion of the spectator - but that is unlikely to be sufficient for the reasons above.