There is a common confusion as most are actually reversely threaded. So the arbor nut / bolt will tighten in the way the blade turns when powered. Have a quick look at how your mitre saw spins when it is powered and then go in the opposite direction to loosen the bolt.
If still stuck you could try heating the nut up with a pin flame just enough to loosen. Just make sure that you do not heat it up too hot that it will melt plastic and damage the saw itself. - Noah
If it’s not moving, try using multi grips on the sides of the flange. That works with my grinder. - StevenSee
I have two mitre saws and both are quite old and rusty as I keep them outside in an undercover workshop. Mine have two different fixings. One is an allen key and the other is a standard bolt. Both unscrew by turning them clockwise which is the opposite to a normal bolt. First, you need to ensure the power is off and you have removed the plug from the power point. You then need to lock the blade in position and I achieved this by placing a piece of timber under the blade and pressing the saw down onto it. For the allen key version, I cheated by using my impact driver and after about 20 secs of struggling it unscrewed the allen bolt. On the bolt version, I removed the guard on the side of the machine, attached a spanner to the bolt and tapped it with a hammer. This broke the bolt free easily.
If you are unscrewing the bolt in the correct direction which is clockwise and you are struggling to remove it, then I would suggest returning to a Bunnings store and seeing if they can assist. They will assess the machine and work out what would be the next course of action. - MitchellMc