I think I misunderstood something here.. If someone pays a small amount to be able to darksend for 24hours, then it means those people could mega spam during 24 hours, regardless of the MNs, no?

EDIT: or you meant people pay for the time the anonymization process? Then they actually darksend 10 days later for free if they want to? So if it's about anonymizing the funds, then yes people can't spam or DDOS