You can just turn off the Wifi on your DSi and then never upgrade the firmware. The only problem is that future games might require certain firmware versions to operate. But for the 2000+ DS games that have already been released then you'll have no problem playing them.
Edit: Make that 3600+ DS...
I did a quick google search and I think the latest version is 1.2J. Can anyone confirm that this is OK with the Acekard 2i?
Edit: According to some people, 1.2J is fine with the AK2i. Just want to know if anyone has personally confirmed this?
I have a DSi with 1.0J firmware on it and I want to download some apps from the DSi store but you need to update the firmware to access the shop. What's the newest firmware for the DSi and is it ok to have it with my Acekard 2i?