Well someone asked me about this the other day, so i thought i would post it and share with the world. It is documented but I remember when I first set the internet up on my Nintendo DSi I had a job locating this information.
If you have downloaded the Nintendo DSi Browser and you would like to restrict access to browsing the internet, this can be done through the Parental Control settings of the system.
Note : When you go to the Nintendo shop you may be asked to update your firmware, be aware of this if you have a “card” (i.e R4, TT or AceKard etc) as this can stop it working and you may need to patch your card after (as I had to do this the first time with a AceKard)
More information on the Nintendo DSi browser here