I don't want to keep pressing the "5" key on the keyboard to "add coins"
Start a game, press tab, select "Input (general)", select "Other", and
remap coin 1 to a button on your control panel (or gamepad, or whatever you're using). Then you don't need to use your keyboard.
is there a way to disable this feature?
"Disable" how?
Change the default buttons from the "5" key to something else? Yes. (see above)
Disable emulation of the original arcade game's game play? No. (mame is about emulates as much as possible as close as possible)
Use the original game's freeplay (if it had it)? Already does.
Add freeplay to game's that didn't have freeplay? No. (that involves either hacking the original game, or disabling emulation mentioned earlier.)
And if you don't have a enough buttons on your CP/gamepad/controller, just map a button combo.