This KB describes how to force the K2 server to use a specific Domain Controller.
You will have to modify the LDAP string to use the Domain Controller you want to use, as described in this article:
Build the string as follows:
Where DC1.domain.com is the domain controller, that needs to be used.
Go to K2 Management then browse to your Users > K2 > Domains where you should see all your registered domains with their respective LDAP:// or GC:// strings.
Edit the LDAP:// or GC:// strings there to only use one Domain Controller as per the above.
Please restart your K2 Service for the changes to take effect.