For moderately tech-savvy people who are planning on deleting their LJ accounts, I have a script that will first go through and "burn" each post and comment by replacing its content with junk. *Then* you'd want to delete the account. This is potentially more thorough than simply deleting. (It does not remove your content from their backups, of course, but it prevents them from merely soft-deleting your journal and posts.)

This is obviously a pretty dangerous tool, so use it carefully: https://github.com/timmc/lj-expunge

(I haven't tested it in a while. I assume it still works, but no promises, and I won't be able to offer support. The script is offered as-is.)