Silly post - but entertaining.
I have 100% been where you are with my T-Mobile account. My brother would notoriously pay his bill late even with constant reminders. To be honest, the best remedy for this would be to have the lines suspended on the day the bill is due if they don't pay their portion. This is tricky though because if you are the primary account holder, you are ultimately responsible for the entire bill no matter who uses the lines. Also, if you let those lines do upgrades on installment plans and they still have money left to pay on those phones, you are responsible for those costs too.
In my opinion, if they continue to pay late and there is not phone payment plan associated with their line, suspend it on the bill due date (you can call us to suspend for lost/stolen and it will shut off the phone) and if they still don't pay, cancel the line altogether. We don't have service contracts so you would not be penalized for canceling the line. I only mentioned the phone installment plans as you would still be responsible for those.
As a side note, if you cancel a line on the account, any promotions associated with that line if you had any would be removed from the account and could not be added back.