To disable the hud partially, press the minus key "-" to shrink the hud down to only the health meter.
To disable hud completely, open the console with ~. Type "hidehud".
Each load will reset the value, so it's not permanent. To make it easier, bind a key to the hidehud command.
While in game, open the console and type:
bind h "hidehud" (replace 'h' with the key of your choice). The binding will hold with game reloads and game restarts. To disable the hud, just press the 'h' key each time. To re-enable the hud, press the '=' key.