If you stop exim from directadmin it will not auto restart. It will update /usr/local/directadmin/admin/services.status to exim=OFF so that it wont try to restart it. But you should also probably chmod 0 /etc/init.d/exim
I dont know if there is an official way to disable it or not.
The reason we disable exim completely by renaming it rather than by chmodding it is to allow us to easily search for all disabled software by doing a find on *.off.