Currently this can only be done via mods. As kju mentioned on Arma 3's Discord: "cfgActions can be modified via configs but not via sqf".

For my particular specific needs I just need the self/friendlies/enemies heal action that appears on self, or when aiming at wounded friendlies and even enemies but I can see other mods and mission makers benefiting from a script command to remove other/all default actions as well.

Example mods:
No Actions by johnb43
LINK: https://steamcommunity.com/sharedfiles/filedetails/?id=2126300184

ArmaForces - No Actions by veteran29
LINK: https://steamcommunity.com/sharedfiles/filedetails/?id=1682845363

Currently mission makers have to force their players to download mods just for removing some of those default actions.

Forcing players to download yet another mod quickly becomes a problem for mission makers who want to make vanilla/DLC missions without extra/additional mod requirements or for missions already dependent on too many mods or for mission makers trying to avoid potential "modding incompatibilities" with A3's future patches/other installed and enabled mods, single player missions etc.

The main reason for requesting such feature is that Arma 3's Default Actions such as the heal wounded action that appears on self, allies and even on wounded enemies can cause interferences/duplication with custom made (Scripted or modded) Medical Systems, Melee Systems and/or other scripted/modded alternatives for many of the default Arma 3 actions and interaction systems.

Other reason to include this functionality would be uncluttering the Action "scroll Wheel" Menu.

Currently some Action Menu items that pop up for example when having different ammo times for your current weapon etc can end up cluttering the already cumbersome/cluttered Action Menu.

On some missions in particular where the action menu is already pretty cluttered can cause players to miss some key actions set by the mission maker and/or just make the scroll wheel menu quickly become really panful to use.

Due to all that was stated previously I believe it would be super helpful to let mission makers disable these actions at discretion via SQF commands or perhaps another vanilla solution that requires no additional mods/config tweaking.

PS: The following are just examples and by no means represent the real/complete list of actions nor the extent of this suggestion (That extent is entirely up to the developers' discretion).

showDefaultActions[] = {
1, Heal Action on wounded Friendlies
0, Heal Action on wounded enemies
0, Heal Action on Self when not incapacitated
0, Heal Action on Self when incapacitated
1, Reload Weapon with XX type of Ammo
1, etc
1 // etc
};

Update based on the discussion on Arma 3's Discord Feeback Tracker Channel:

kju also suggested: "Probably better would be a way to disable them for entities altogether and script the still needed actions instead"

As Dedmen asked how ACE removed some of these actions [KP] veteran29 replied: "we change item type of FAK to not be a FAK anymore".

All relevant participants including but not limited to Leopard20, Sparker, kju and of course [KP] veteran29 suggested a scripted solution would indeed benefit their works/mods significantly. (They would no longer have to rely on workarounds to achieve the same result).

All the community members involved in the discussion on A3's discord server also acknowledged that they consider that having such functionality would be very helpful for mission makers and even modders in general.