@Helga Mayr - This is not currently possible. The only workaround that comes to mind is to use the Board Co-owner feature, which comes with a few (tedious) caveats:

1. It is only available for Business and Enterprise Plan subscribers
2. for every board where a team member may need to unlock something, but Guests or Visitors should not, the board owner would need to add every team member who should be able to unlock objects (frames included) as a Co-owner of the board. The board owner would also need to apply a Protected lock to every object that only the Owner and Co-owner should be able to unlock.

Wish List idea post

There is an existing Wish List idea post asking for something similar that you may want to upvote: https://community.miro.com/ideas/disable-object-unlocking-for-guest-editors-998