Question about B2 visa extension! xyxx2008-10-09 15:35:50

I have a question about B2 visa extension, I looked at pjiang blog and didn't find an answer there.

If the B2 visa holder go to Canada before the I94 expired. When they return to US, do they must wait until the I94 expired to get a new I94? Or could they return the same day or the next day within the I94 expiration day and also get a new I94 for another 6 months?

Does the US custom collect the I94 when they leave US? Can they deny their entry back to US for any reason? They do have a valid visa.

Thanks a lot!