This theorem is an immediate consequence of a result by B. Maskit, which states that one may associate with $\Omega$ another domain $\Omega'$, conformally equivalent to $\Omega$, such that all conformal self-maps of $\Omega'$ are Möbius transformations. For a proof of Maskit's theorem, I suggest you look at the following papers :

- Maskit, Bernard
The conformal group of a plane domain.
Amer. J. Math. 90 1968 718–722.
30.45

or for a more elementary proof :

- Peschl, Ernst; Lehtinen, Matti
A conformal self-map which fixes three points is the identity.
Ann. Acad. Sci. Fenn. Ser. A I Math. 4 (1979), no. 1, 85–86.