Licenses are automatically removed after 60 days for inactive endpoints. If you don't want to wait that long, there are several other ways:
You can uninstall ABR from an endpoint - the license is instantly recovered.
You can find the endpoint in the Inventory and use either the Delete menu (at left) or the DELETE tab to remove the computer.
There is a bulk delete capability available using API calls, described here: https://www.adminbyrequest.com/en/docs/inventory-api. A video is available showing how bulk delete can be done in Postman: https://www.dropbox.com/s/nsoyi0crma63y4i/Multi-Inventory-ID-Delete.m4v?dl=0.
For more information, refer to Licensing.
For Frequently Asked Questions about licensing, refer to All FAQ (Licensing).