Just as volunteers aren’t obliged to help your organisation, your organisation is not obliged to offer a volunteering opportunity. You can’t fire a volunteer in the same way you can an employee because they haven’t signed a contract, therefore they can’t be in breach of its terms. However you can inform a volunteer that there is no longer a volunteering opportunity for them at your organisation.
Asking a volunteer to leave should be a last resort. However, if you’re considering asking them to leave simply because the role they perform is no longer needed, then you should think about other ways that they could help out and offer them alternative roles. If there is an issue of performance or conduct you should think about whether this could be addressed and the volunteer given the chance to improve. It may be as simple as expectations not being clearly communicated. If it’s an issue of capability then there may be ways you can support your volunteer to improve their skills.
Volunteer Scotland has more information on managing challenging volunteer situations.