@Jenny Pither - Welcome to the Miro Community!
In a Free Plan team, anyone with the Team Admin role can remove both a regular team Member and a Team Admin from a team.
However, if you are saying there is only one person with the Team Admin role, if you are unable to gain access to that account to promote another Member to Team Admin, then you can always create a new Free Plan team, add the existing users to that new team (and make more than one person an admin!), and then move the boards from the other team to the new team. Full steps in this post under “Creating a new Team Admin (via a new Free Plan team)”:
https://community.miro.com/miro-labs-113/taking-back-control-of-your-free-plan-team-7706