Asking a volunteer to leave should always be a last resort when other ways to solve a problem haven't worked. See our guidance on solving volunteer problems. In rare cases, it's a necessary response to a serious breach of conduct.
You do not have a contract of employment as a volunteer, so you do not have the same rights as an employee or worker. You will usually be given a volunteer agreement that explains: the level of supervision and support you'll get. what training you'll get.
An employer cannot require you to volunteer and work unpaid hours unless it falls under your job description. If the volunteering is presented with the threat of adverse employment action if you do not comply, then you may have a claim for unpaid wages.