A "no-fly zone" means no flying is allowed by the forces against which it is directed.

The first logical stage in enforcing this is proper monitoring to ensure that it is being observed.

Only if there is evidence that it is being ignored is it appropriate to send up aircraft to enforce it, and the matter of ground-missiles etc become relevant.

Otherwise it's a bit like telling bank robbers to come out with their hands up, and shooting them when they do.