Who do I report a foriegn flag flying on a flag pole at a house nearby?

The Secret Service or the FBI.

Local Law Enforcement MIGHT do something about it.

Also, it really depends on whether the country is an ally of the US or not. If it's an ally then most likely nothing will happen. If it is a communist country, they WILL remove it.

For example, how often do you see Chinese or Cuban flags flying?

Make sure your site the EXACT Law when you speak with the FBI or Secret Service and BE NICE.

But also, don't expect much if it's an ally...

You can report it to anyone that will listen, however the flag code is not law and the government has no authority to make the person take down the flag.

I thought this was a free country. I love Japan. So does this mean I can't fly my Japanese flag? I just love Japan. But I'm not a terrorist against my country.

You asked this already and the answer is the same. It is not illegal to fly the flag of another nation at your house.

You can't be serious...The constitution says that the US flag shouldn't be used as clothing but i bet you are wearing "flag" boxers right now. What is more disrespectful, displaying another nations flag or letting ours cradle your junk?

But it's not a crime to put another flag up. So, there's nothing to report.

You face the same criminal penalties for flying a US flag. None.

Unfortunately no one enforces the law. You could try calling your local FBI field office and present them with 4 USC § 7 and if they're in the mood technically they could enforce it.

Relax

Why report it when it is likely all you have to do is advise them (politely) of the proper protocol.

if they were a threat, they probably would not be advertising that fact.

Free Speech. Mind your own business.