I just checked all my active campaigns and found none have any clause about multi account LOL. It's a so common practice on the forum that I never thought to put something about it before, but now I see there is a need or people will use it as a hole. I took a note for myself to add the clause from the next payday of the campaigns. Thanks for pointing it.
It is understood that Multi accounts are not allowed in any campaign whether you write it in the rules explicitly or not. However, having written in the rules gives the manager an edge as the applicant can't challenge that there was no such rule in the campaigns and therefore, they enroll their alt accounts.