Delete one voice or one staff from a system

Place the cursor on the relevant voice. Select menu edit | delete | stave/voice.

Repeat this step, until the last voice has been deleted. Together with the last voice, the relevant staff will have been deleted. Only once this stave has been removed from the entire score, can you remove it from the relevant template system as well. This is intended as a safety feature, to prevent your score from getting corrupted.