Lock & Unlock Cells Based On Validation

I need to lock certain cells on a worksheet when a value is selected from a validation list. For example:

user goes to select an option from a validation list, that cell is now locked.

User decides to change the value with a blank option, available on the same list. the cell becomes unlocked ready for new input from the list.

the cell can only allow input when the value in the list matches that of another cell. if another cell already has different data in, it wont allow you to change the value.