Open a board's ... menu → Permissions
Choose who can edit the board: everyone, board owners only, or specific people/teams
At the workspace level, click the workspace name → Manage members to control who can view or join the workspace
Default permissions are usually too open
Newly created boards often default to editable by anyone in the account. Review and tighten permissions on any board with sensitive data (salaries, client contracts, etc.) as soon as you create it.