On the starting map, make a parallel process event with the following script call
$game_system.hud = false
$game_system.hotkeys = false
$game_system.minimap = 0
$game_system.hud_button = false
$game_system.hotkey_button = false
$game_system.minimap_button = false
That will hide everything in the hud, and make it so the player can't turn them on. Make sure to, when you want the hud to function normally, execute the following code in a script call
$game_system.hud = true
$game_system.hotkeys = true
$game_system.minimap = 1
$game_system.hud_button = true
$game_system.hotkey_button = true
$game_system.minimap_button = true
If you want a list of all these functions, look in 3.2 utilities in the blizzABS user manual.