How to lock a field value using variable

[sakshikaul]

Hi 

I am interested in locking a field value with the help of a variable. For eg if I want a selected filter has two values 'yes' and 'no ' and I want to give users only  single option (NO) to select the other value I want to lock/disable 
How can we achieve this?

I want to the user can only select 'No' for flag field and other option ie' YEs' should be disabled for selection. How can this be achieved

?

[sakshikaul]

Have you selected onLock option and then added an action?

[sakshikaul]

I have tried implementing the sheet level triggers but still its not working