What can I do to stop imessage from engaging on a certain contact without an iphone?
Nothing. If your friend hasn't deactivated iMessage before he sold/lost his iPhone, then Apple still thinks that he has an iPhone and is using iMessage. 

He has to deactivate iMessage. Here is the Apple Support site for this.


  Bun in short: He has to contact Apple Support to do this.


Or, when he/she still has his/her iPhone, then go to Settings > Messages and switch off iMessage.