Uggggghhhhhh. Yeah, we haven't had to build in stronger automated spam cleanup tools than the ones LJ had in '08 when we forked, because we haven't been a tempting enough target for spammers that the spam volume has increased past the point where our antispam team could handle it themselves through those tools. :(

If you don't want to hand-delete all of the spam, you can delete all your imported entries, go back to LJ, use their cleanup tools, and then re-import. (I think it will also work if you delete just the entries with the worst spam, go over to LJ, use the cleanup tools, and reimport; I'm pretty sure the importer will recognize that the post is missing and re-create it, and even if I'm wrong, the failure state would be "fails to re-import the post(s) that was deleted, requiring you to wipe everything and re-import if you want to make sure you got everything", not "duplicates everything". I'd read the code to be 100% sure instead of just pretty sure, but it would take me too long!)