@Kristian - Yes, the board owner/co-owners can use Protected Lock to stop all other participants from unlocking content.

To further clarify, there is no role of “host” in Miro. This is important to understand because, just because someone is hosting the Miro board session, this does not mean they are the owner of the board. Sometimes one team member may create the board, but another may lead a session. If this happens, the board owner should grant the other team member the role of Co-owner for that board.

More on this in the following Help Center article → Roles in Miro.