Can I tell my tenant not to be sitting on porch stairs?
You can not control activity such as this unless there is very specific language in the rental agreement (the contract between renter and landlord) that prohibits said behavior or it is a violation of a local ordinance like a building or fire code. If only one person is sitting there, chances are no violation to either exists.
That being said, the tenants behavior and where he chooses to sit, in this case, are solely a decision he makes and you as the landlord have no recourse.
On the other hand, if this act of sitting in this particular spot is a nuisance or safety concern for other tenants in the building, then you could call local law enforcement authorities and they could compel him to stop the behavior. Aditionally, if other tenants complain about this behavior, then that to is a matter that is best suited for the local police.
From my short experience with leasing, I believe you are going to run into trouble unless there is language suggesting that sort of thing in the signed lease document. Basically most leases give the tenant control over entering and exiting the property and deciding where they will or won't sit at any given time. Unless you specifically put language that prohibited sitting on the porch stairs in your lease, it seems unlikely.
Plus small-claims courts, likely the place you would have to take up this issue, has often been biased towards tenants over landlords so I would be careful about pushing it if I were you. Asking nicely might not hurt, but saying it is a rule, etc., could get you in trouble.