How to delete/edit a record where a field matches a value?

I'd recommend creating a joined table.

Basically you will have a "Locker Assignments" table.  Each Client has many assignments AND each Locker will have many assignments.

Then within the assignment you can have start and end dates, and easily be able to tell if a locker has an "active" assignment.

That will also drive your reports and drop-downs for "Available" lockers.

Then you will also have a history of locker assignments, whereas right now it looks like you lose that when you change the current dropdown connection.