Wiktionary
throw away the key
vb. To never release someone (e.g., from prison).
Usage examples of "throw away the key".
Goddamn drunk drivers, they ought to lock 'em up and throw away the key.
They'll lock me up and throw away the key, and in the process I stand an excellent chance of getting dead.
Until that point, facing the householder and the householder's gun - and so much for all those chapters the householder was supposed to be away in - and then facing the local law, Dortmunder had fully expected to go forward from here in a normal fashion, through the program, all hope gone, three times and you're in, throw away the key, okay, ya got me, I'll never live on the outside again.
Gideon Guthrie's been singing for his supper and believe me, they're gonna throw away the key!