Is multirate signal processing involved in ADSL? Telecom companies offer different data rates for home/office purposes depending on the money they pay. 


How are the telecom people able to control the flow of data of different cables?
Say supposing a cable can be used till $20\textrm{ MBps}$. But he paid only for $5\textrm{ MBps}$ scheme, why is the user not able to steal the remaining bandwidth? 
Is there a multi rate theory behind this? 
Or it is a simple filtering?
"The telecom people" are the ones providing the internet data to you. Hence, they can easily throttle data at their end, making sure that you do not get faster access than you pay for. If the actual signal processing is adapted based on this I would say depends on the solution.