The same as if you lost the lock-screen passcode. You have to restore the iPod via iTunes on your computer. If it askes for the unknown pass code then you have to place the iPod in recovery mode and then restore. For recovery mode see:
Let us know if you had to place it in recovery mode since the Apple instruction are unclear if it is required for restrictions passcode.
Thank you very much for helping.
Actually, I found my code back yesterday evening (I tried several possibilities, like credit card numbers, birth year of relatives etc. and eventually I hit the correct one :-)
I found this makes an issue, though. It is incredible that a 300 $ device should be blocked so easily. I think Apple should consider the option of providing its iPhone/iPod/iPad users with a PUK code enabling them to find their four-digit code back when lost. It shoul be easy to provide such a code based on the serail number of each device.
What do you think of this suggestion ?