hey
you can disable the crosshair by clicking
configure > game > difficulty > weapon crosshair and change it to disable

Thank you, I made it now but still the same. And I wanna do it on the server too for all players.

weapon crosshair in arma 3 keep on.

Choose a higher difficulty then. Like Veteran or Elite. There might be a mod that totally disables the crosshair but you'll have to search.

I heard about this WeaponCursor = 1 line; so switch to WeaponCursor = 0; but do not know where to place.
//
class Difficulties
{
class regular
{

		class Flags
		{
			Armor=0;
			FriendlyTag=0;
			EnemyTag=0;
			HUD=1;
			HUDPerm=0;
			HUDWp=1;
			HUDWpPerm=1;
			WeaponCursor=1;
			AutoAim=0;
			AutoGuideAT=0;
			3rdPersonView=0;
			ClockIndicator=1;
			Map=0;
			Tracers=1;
			AutoSpot=0;
			UltraAI=0;
			UnlimitedSaves=0;
		};
		skillFriendly=0.850000;
		skillEnemy=0.550000;
		precisionFriendly=0.850000;
		precisionEnemy=0.490000;

};
class veteran
{

		class Flags
		{
			HUD=1;
			HUDWp=1;
			HUDWpPerm=1;
			WeaponCursor=0;
			ClockIndicator=0;
			3rdPersonView=0;
			Tracers=1;
			UltraAI=0;
		};
		skillFriendly=0.850000;
		skillEnemy=0.850000;
		precisionFriendly=0.850000;
		precisionEnemy=0.850000;

};
};

SPAM! Stop spamming gentelmans, there is forums for it, this is place for BUG REPORTING!

As has been suggested use difficulty settings or showHud false command. You might find a better response to your query on official forums. Closing as not a bug.