There isn’t a way to clear a column in multiple rows at once. At most you can clear a value through a single relation to only one row.
I would consider a different approach. Essentially you need to set a Unique ID in a table, such as a user profile table. Then create a Single Value column or a Template column that retrieves that Unique ID from the user table an populates it across all rows in the Ovelser table. Then whenever you you mark an item as completed, write that unique ID to the ‘Check Eller ej’ column. Then you can check if both Unique ID’s match, and if they do, set the checkbox emoji. When you are ready to reset, then you change the Unique ID in to the user table. I go into more detail in the post below.