Actually, it depends on the size of the ice and fire technics. Fire bullets or fire spears for example are definitely easy to stop with some big ice technics. That means fire can be stopped by ice in some circumstances.
I think that even the hiken could be stopped
with a big harsh ice wall because it would not instantaneously pass through the ice.
Stopped does not mean vanished : the fire does not have necessarily to be put out to be stopped. If the fire meets an obstacle - an ice technic - and does not melt it instantaneously or almost instantaneously, his trajectory will be affected and it will stop, even if it does not disappear and even if the ice is at the end totally turned into water.
This
page, shows clearly how the wax, even if it melt, was useful to block the poison of the Hell's Judgement that melts everything. As you see, the poison of Hell's Judgement did not disappear, it was just stopped - because it had not instantaneously melt the giant candle wall - and Luffy, Mr 3 and the others were not touched by the melting poison even if finally the giant candle wall was melt.
To sum up, only fire that burns instantaneously or almost instantaneously ice will not be affected by an ice technic and will continue his trajectory.
That means if Ace does a fire attack with concentrated fire - that looks like lasers or Ki Zaru's concentrated light - it will necessarily melt instantaneously the ice and it will continue his trajectory with no problems. For example, if the
crossfire meets a big harsh block of ice, it will just melt instantaneously the part of the ice touched and will continue his trajectory normally. This is the same thing with the
Entei, it will not be stopped by ice as it melt it instantaneously.
But as I already said, the common fire - I mean the one with flames - can be stopped.