The last bill that was refused assent was the Scottish Militia Bill during Queen Anne's reign in 1708.
In Australia, my country, presenting law for Royal Assent is a rubber stamp, a symbolic act that transcends the current elected government houses.
Should "Royal Assent" be refused by the Crowns agent in Australia you'd see the country exit, the Crown is there for show, not to get involved or to meddle with the work of the elected Government.
I suspect Canada, New Zealand, et al are similar.
You could always point to an instance in which the Crown refused Royal Assent if you'd care to.