Workflow Doubt

hi experts,

I need to introduce workflows in an organisation where i'm working.From PO till MIRO,the condition is,if PO is created then a mail should go to concerned person for allowing other person for doing MIGO and so forth.My prb is how can we stop the migo user from doing transaction even if the concerned person has not permitted him to do so via mail.There should be a system taking care of the actual flow introduced in Worklow. I heard from somewhere that this settings is done thru SPRO.Please guide where these settings are made.

Thanks,

Gaurav