Can dementors actually be killed?

[dragonkeeper]

No: HPFGUIDX 182398

Someone in the past must have found a way to control the Dementors 
with a charm or spell at one time and then figured a way to use 
them for the benefit of the witch world and if they figured a way 
to control them then they must have found a way to kill them.
   
My question involves how the ministry can control the Dementors? 
What do they use to control the Dementors and if they know how 
to control the Dementors then they must also know a way to kill 
the Dementors should things go awry.

dragonkeeper