"Fan: Ignore GCODE" menu option

Sometimes while printing I see some sagging and I want to turn the fan ON all the time.
You can do that on the Marlin Menu TUNE. But when somewhere in the GCODE the fan is told to shut off.. then the code wins.
I need an option om my Marlin Menu that tells the printer to ignore FAN gcode and let the speed I set be constant.
I know how to add menus to the Marlin firmware... My problem is the best way to code the "ignore GCODE for fan" part.
Any help?
thanks

grep -v M106 input.gcode > output.gcode

removes all fan commands

might also need to remove M107 (obsolete, but still used fan off)

Its just text... why make it harder than it has to be....

Thanks Dust, but that is not what I'm looking for.
If i wanted to change GCODE on the file, I would just take that option on slic3r before export.

I need it at mid print, if I see some sagging and I want to turn the fan ON all the time.

How about a little Hardware-Hack?
You could choose between the Fan-Pin and +V directly if you soulder in a simple 2-Way switch. It might not be the cleanest solution, but it works.

Being a hardware guy myself, that was my first thought. 3 position switch: GCODE mode, OFF, and Always ON.
But then I realize... I have a cute LCD screen with menus, why not try to do it right?

Hardware option will be failsafe go to. But I was hopping for some firmware wizards to help me do it the right way

redefine fan pin in pins.h to -1 unused

Then turn it on/off with the apropriate m42 command.

this will stop normal gcode touching the fan and there is only manual fan control.

(also not quite what you want, but is simple to do)