Major Drug Org. -- SCREW KITSON ... We're Not Taking Donations from Rx Shirt Sales

One of the biggest anti-drug organizations in the U.S. has drawn a hard line in the sand -- telling a trendy L.A. clothing boutique it will NOT accept donations collected from the sale of "irresponsible" drug-inspired shirts.

The shirts in question have been at the center of a media firestorm -- featuring the names of various prescription drugs like Xanax and Vicodin plastered across the back.

After taking all sorts of heat, Kitson -- the boutique selling the shirts -- has said it would donate part of the profits to the Medicine Abuse Project ... a partner of DrugFree.org.

But DrugFree.org is pushing back HARD -- firing off a letter to Kitson saying, "We would not entertain any direct donation from Kitson while you flagrantly, and without remorse, continue to sell these products."

Translation -- take your money and stick it.

DrugFree.org also demanded Kitson remove the org.'s name from the store's website -- and Kitson has obliged.