Update:When it comes to mass deletions, I had to hunt around quite a bit more. I couldn't find any standalone tools, so I started looking at libraries. Two (TWO!) perl libraries had gone out of date, but I found a python one that was still working. It hadn't implemented the one lj API command required for deletions, but it had everything else, so I forked it, added the necessary code, and wrote my script.
The forked python-lj library is here: https://github.com/krivard/python-lj
The delete-my-livejournal-entries script is here: delete-lj-entries.py
Along the way I found a slightly better archive/backup solution that saves to XML instead of .plist (superior to ljkit) and also saves comments (far superior to ljkit): lj-dump 1.5.1
Just to be clear -- back up first. Raze the ground behind you second.
See yinz on dreamwidth, or elsewhere on the 'net.