I am not a US citizen or permanent resident of the US. I bought a house for my kids who were studying in the US. The first time I bought apt had slight problem because I did not have a social security number. My housing agent suggest I get one but I cannot because I am not a US citizen, so he suggested getting an ITIN. I got it. Now my kids have graduated and went home, also sold my apt. I don't need the ITIN anymore so should I send it back to the IRS?

I do not think that you should send it back. Just keep it with you and do nothing.

Agreed! If no tax documents are filed using the ITIN, it eventually becomes inactive and then is re-assigned to someone else.

You CAN send the ITIN memo back to the IRS and request it be revoked, but it is NOT required.