In general terms, if it is a restricted POA (created for a single task such as selling/buying a property and this is detailed in the POA) then once that task is completed the remit of the POA is completed and as such, its not much use to anyone.
If it is a general POA, which I suspect is the more common, then its a pretty powerful document, especially in Turkey. You need to go to the Notars office where it was issued from and cancel it there. They inform the person with the POA that it has been cancelled - I think they also inform the local deeds office but not 100% on that.
Its a common procedure and you'll need an official translator. No idea about charges as never had to do this.
JF