It may be worth enabling Spam Quarantine just for this spam cleanup. I imagine you can disable it again once you're done and are considering your long term options.
I used to manage a large community that would get hundreds of spam posts a month -- thousands of spam posts during "attacks". The Spam Quarantine definitely caught a lot of false-positives, but the data always showed that it was worth it in comparison to how much spam was caught.
The addition of the "your message was flagged as spam" notification was great for helping new users understand what happened. I also set up a "whitelist" role that I could manually apply to new accounts when we saw them get flagged as false-positive spam. You can also build that whitelist role into your ranking system. I also added the "bypass spam" permission to employee and super user roles.
A handy tip in addition to the spam filter for 'cleaning' accounts that didn't submit spam, but spammed multiple topics etc.
- Do a blank search followed by entering their username in the Author option.
- Click Advanced, ensure View results by is set to Specific posts
- Click the options button and turn on batch processing
- Run the search again, click any post and use the Options menu to move in bulk
