bbPress.org

The information you got was probably for the bbPress v1 standalone forum software. You can tell the information is from bbPress v1 when it is about 4 or more years old.

The latest version of bbPress is now a WordPress plugin. So any admin area would use the WordPress admin. Same thing with BuddyPress they use use the WordPress administration area.

To remove a large number of topics.

Go to Topics > All Topics in the WOrdPress administration area.
Enable each checkbox by the topics you want to Trash
Click the Bulk Actions dropdown and select Move to Trash
Hit Apply to move all those topics to the trash.

To remove a large number of topics in a specific forum

Go to Topics > All Topics in the WOrdPress administration area.
In the dropdown at the top that say In All Forums, click that to select the specific forum you want to delete topics in, then click the filter button beside it.
Enable each checkbox by the topics you want to Trash
Click the Bulk Actions dropdown and select Move to Trash
Hit Apply to move all those topics to the trash.

To delete all the topics in your topic trash.

Go to Topics > All topics in the WordPress administration area.
Hit the action link that says Trash, it is next to other action links in a row like Spam and Pending.
Clicking the trash action link will show you all the topics in your trash, hitting empty trash will remove all topics in your trash permanently.

To trash all topics by a specific use

You need to go to the user’s profile
Click the menu item Topics Started
Go to each topic and trash it manually using the admin links.