Admin/content allows you to filter the listed nodes. I don't use any forums right now so I don't know that they produce a "forum" content type but that certainly would simplify the task.
Also, you can select multiple nodes (the check boxes on the left) and perform the same operation on multiple nodes using the Update tool toward the top of the list. One of the update options is delete.
Good luck; hope this helps,
Curt
On my new site, I inadvertently allowed anonymous users to post forum topics sans spam filter. Now there's something like 6000+ spam post. Is there a way to just wipe out all forum content? (There's nothing I need to salvage; it's all spam.) On admin/content, it looks like I can delete one page at a time, but with 120 pages that would take forever.
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