Just do what Bulgaria did a few months ago - Give them a free single ticket to Europe - on the condition that if they return, they will serve the rest of their sentence.
Worked for them.
The thing that gets me, is that in the UK we actually have prisons specifically for foreign nationalities - not british inmates. Why on earth should we pay for the upkeep of foreign prisoners ?
Years ago - there were traded just before winter, as food was scarce, and it made sense to return them - Bring this system back I say!