Need to block certain countries or not?

Depends what your site is. For online gambling, as an example, the US does require that you prevent people located in the US from accessing your site. I don't know what the rules are for file sharing, but they have been cracking down on illegal file sharing (however they define that) in recent months.

The harder question is how to go about doing that - if it's done strictly on IP, people can spoof that easily. At the same time, if you make an honest effort to comply with their laws, then you should (speak to a lawyer first) be off to a good start.

I block certain countries for personal and security reasons. When user is blocked a polite message is shown. The way i do it is to check for some Accepted-language in web server configuration. I realize that this is not the best way but is working perfectly so far.

As Elie said, first of all it's really about your content. If it has illegal content, you are better off quitting either way.

Furthermore, if a certain form of file-sharing is illegal in a country, it's the user's responsibility. An IP follows a certain standard. As so, it is easy to recognize certain country's by looking at the IP. The best thing to do is not to block IP's, but simply add a warning that says something along the lines of:

"This is a file-sharing website. Your country might not allow the use of file-sharing websites and/or any such content.

Use this site at your discretion."

discretion [dɪˈskrɛʃən] n

1. freedom or authority to make judgments and to act as one sees fit (esp in the phrases at one's own discretion, at the discretion of)

Actually it does not depend on USA law, it depends on where you live. Chances are if you live in a country such as Panama or the Turks Islands then you dont have to block anything. If you are hosting items that the USA government finds objectionable or illegal, they will eventually do the blocking for you.

If you are a US citizen, then you are subject to USA law even if you live outside the USA. Meaning you simply cannot go offshore to run a company that would be illegal in the USA to a USA audience. There is a dirty method to this which includes incorporating using an agent (not the best idea, using barer bonds, and trying to convince the USA that the majority of your clients are not in the usa)

Last, if you are in a country that has a close rationship with the USA then you risk getting in trouble there.

After doing a few "Adult" projects, i learned quickly that the real money is in B2B. The profits in Adult and gambling are enticing, but the due diligence and legal burden is often not worth it. By the time everyone gets their piece of your pie you end up with a lot less, and more liability.

If you have not started your project, invest some time investigating alternative projects to move forward with. If you are committed still to this project, I wish you the best of luck. I can personally recommend you speak with Lawrence G Walters law firm in the USA regarding 1st amendment and copyright issues. Few lawyers practice internet criminal law.