kyanar

No, no it is not. You can't just take a third party's property. In any country which actually follows the rule of law, that's unlawful. If you owe money to a finance company, and they get a court sheriff/bailiff to come to your house and repossess things, they cannot just take random objects belonging to other residents or rented from another company.

EDIT: Done a bit of research on the background. In Isavia's Terms of Service, clause 3.6 purports to claim the right to prevent departure for various reasons, including "Airport charges or other charges related to that aircraft or its operator have not been paid and an acceptable guarantee for payment has not been provided". This seems to be backed up by Article 136 of the Icelandic Aviation Act.

The tough part is that legally, in the case of an insolvency, what happens is the company effectively ceases to exist, and begins operating as a new entity (most countries represent this by appending terms such as "(In Liquidation)", "(In Receivership)" or "(Under Administration)" to the name). This pseudo-entity is not only not liable under law of the country it is incorporated in (important note, since as Isavia points out German insolvency law is not recognised there), but it is legally not permitted to pay the debts of the insolvent real entity as that violates the priority of creditors. Under German law, Isavia is most likely an unsecured creditor, which means last on the priority list. As such, if airberlin were to pay them, the insolvency administrators could be prosecuted for treating one creditor as priority over another in breach of law.

They literally cannot legally pay it.

I suppose BOC Leasing could terminate their lease agreement and commence repossession actions, but would Isavia allow that? Based on their actions so far, probably not. Meaning BOC Leasing is almost certain to end up holding the bill - one of the issues the Aviation Working Group has been snarling about for the last decade.