It is necessary to delete registration data from digital lock.
This can be done by using smartphone to delete Apple Watch's registration data. 
 

1

Go to "Digital Lock Settings" and select "Manage Registration".

  

2

After transitioned to "Manage Registration" screen, proceed to select "Edit".

3

For iOS users, tap on "⊖" mark on the left side of registered key and proceed to select "Delete" indication that appears later on the right side.
For Android users, tap the garbage bin mark on the right side of the registered key. 
Once above action is done, registration data of selected key will be deleted.

Digital lock registration data will remain in the Apple Watch but will be inoperable. 
For how to delete digital lock registration data from Apple Watch, please click here