49 CFR 1018.62: Reasons for terminating collection action.

Collection action may be terminated:

(a) When it becomes clear that the Government cannot collect or enforce collection of any significant sum from the debtor having due regard for the judicial remedies available to the Government, the debtor's future financial prospects, and the exemptions available to the debtor under State and Federal law;

(b) When the debtor cannot be located, there is no security remaining to be liquidated, the applicable statute of limitations has run, and the prospects of collecting by offset, notwithstanding the bar of the statute of limitations, are too remote to justify retention of the claim; or

(c) When it is likely that the cost of the collection action will exceed the amount recoverable.