Will Ferrell -- Take My Elf-ing Pic Off Your Gun Ad

'Twas the month of November ... long before Christmas begun -- yet a company launched an ad of Will Ferrell ... pushing a deal on a gun.

Poor Will never gave permission for this flyer to be seen ... but there was his beloved "Elf" character ... with an AR-15.

The company which produced "Elf" was especially pissed -- so their lawyers sent a gift ... a cease and desist.

We're told the ad was scrapped ... taken down right away ... out of fear of the cash they soon might have to pay.

So what did we learn from this holiday lesson?