Moving a lead from one nurture stream to another

Hello, I'm new to Marketo and looking for guidance on how to move leads from one nurture stream to another once they complete an action. We use Marketo for our community website, once members register to create an account and select yes to receive marketing communications from us they are put into our new member onboarding stream. 

Of course, we have people in our database that aren't members. We are creating a non-member nurture stream that we hope will eventually lead to them becoming members. 

My question is, once somebody who isn't a member creates an account, then gets put into the new member onboarding stream, how do I remove them from the non-member nurture stream so that they no longer receive those emails. 

Sorry if a question like this has already been asked and answered.