Try the following:
1. Click the blank space in the track once (to deselect any selected clip/track).
2. Click the track number on the track strip to select all clips in the track. Make sure the Clip Name, Start and Length fields in the Clip Properties Inspector say "(Multi)."
3. Now, hold down the Ctrl key and click the top half of all clips that are visible in the track. This will deselect those visible clips. The fields in the Clip Properties Inspector should still say "(Multi)" because the invisible clips are still selected.
4. Select "Off" from the Lock drop-down in the Clip Properties Inspector. This will unlock all locked clips that are selected.
5. Hit the Delete Key to delete all the invisible clips.