You can use a script as stated above and attach it to a dynamic object. You should set the object to always active so that the player doesn't get out of it's range and it stops working.
You can also set the ambience in the build game settings to either not allow any change during game play, or allow an amount that you specify.
If you set the settings as follows...
ambience=20
ambiencered=20
ambiencegreen=20
ambienceblue=20
there will be no adjustment in the game using the <> keys and the ambience will be 20.
If you set the settings as follows...
ambience=20
ambiencered=30
ambiencegreen=30
ambienceblue=30
the <> keys will adjust the ambience between 20 and 30 only.
The ambience is the lower limit, and the ambience r-g-b are the upper limit. You can lock them to the same value, or allow the player whatever adjustment you choose.
Brian.