I don't think there is a command to turn that option on and off. There SHOULD be a command for every option IMHO but until there is you are probably going to have to use raw events to divert the /whois results to your active window. There are plenty of examples of whois scripts out there to learn from. Here are some examples:
Geetee's mIRC script pagesRaw Tutorial by Insides#Helpdesk raw tutorial by B0P 
