While no action can be taken, please do not ignore the letter. You need to respond to the letter. Just as Mr. Jeevarathinam has suggested, please respond to the legal notice. Tell them that no bond was signed,nor were you aware of any such conditions as mentioned in the legal notice. Ask them to send evidence/documentation to substantiate their claims. They will not respond back.
In case you would ignore the letter, the lawyer could file a case. Though the case will not hold good, if you contest it. But, if you do not contest it, an ex parte judgement can be given against you.
Since they have used a lawyer, I would suggest to meet any decent lawyer and respond back. That must shut them up.