Disable registration functionality for sub-domain in Drupal 7
According to this answer on SO (to a 'similar' question posted 'a bit before' this question ...), this is a possible answer to this question (= quote from that question):

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As far as I know you can have different variables per domain (Because the Registration option is a variable) using the latest Domain Access module (currently 7.x-3.11 from 2014-Mar-02). It can be done with a submodule named Domain Settings. Enable it if haven't done yet.

If for any reason does not work for you I would suggest to use the Domain Variable module to handle variables for each domain. 

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