times machine backups

I'm not sure if you want to stop using a particular USB drive as a backup device, or to stop Time Machine from backing up an external drive. In either case the solutions are addressed in Change Time Machine settings on Mac - Apple Support.

Respectively,

• To select, add, or remove backup disks, click Select Disk (if you use just one Time Machine backup disk) or Add or Remove Backup Disk (if you use more than one TM backup disk).
• Exclude these items from backups: Exclude files from a backup to save space on your backup disk, or to exclude items you back up using another method. By default Time Machine backs up all mounted disks. To prevent an external hard disk drive from being backed up, select it, then click the Add + button to add it to Time Machine's exclusions list.

Do not attempt to delete anything from the Time Machine backup. Just add that USB disk to Time Machine's exclusions list, as explained in Exclude files from a Time Machine backup on Mac - Apple Support. If you do that, Time Machine will eventually delete its backed up files when newer backups need the space they occupy.

If you don't want to do that then erase the entire backup disk. In any case make no attempt to delete those old backed up files manually. Doing so will corrupt the backup, and you will need to erase the backup disk anyway.