org-clock-out-when-done

org-clock-out-when-done is a variable defined in `org-clock.el'.
Its value is

t

Documentation:
When non-nil, clock will be stopped when the clocked entry is marked DONE.
DONE here means any DONE-like state.
A nil value means clock will keep running until stopped explicitly with
`C-c C-x C-o', or until the clock is started in a different item.
Instead of t, this can also be a list of TODO states that should trigger
clocking out.

You can customize this variable.