User blog comment:Martialmaniac/How To Steal Your Devil Fruit/@comment-10543621-20150515034748/@comment-5869646-20150515041414

Nearest specific fruit. Otherwise all those illustrations in that DF guidebook Blackbeard and Sanji studied wouldn't amount to anything. For example, the Axolotl fruit's base fruit seems to be an apple, hence Caesar left a bunch of apples near Smiley.

To answer your question about why no one has bothered doing it earlier... Just because we haven't seen it, doesn't mean it hasn't happened. In any case, DF mechanics aren't exactly common knowledge. And you'd have to 1. Know the right fruit 2. Actually have the fruit with you 3. Know who the current user is 4. Track them down 5. Kill them, which, if the fruit is good enough to go through all this trouble for in the first place, probably isn't easy.