Really disabling is impossible. What is possible to flash a non routable US Basemap which you include in firmware. To go back do the same for the european basemap. I once flashed my hometile of OSM. You can put maps up to around 12Mb as Basemap. Doing such things is however very prone to sending your GPS into Walhalla.
will do that when I feel like I need a new one anyway. The Garmin flash process is not very stable and had already troubles 2 times. Once the unit was exchanged under warranty and 2nd time I could fix it with the download version instead the webupdater.
Most of this stuff has been found out by the eastern groups of sellers of imported Garmin units and pirated maps.
There is even a way to flash your serial number (and thereby takeover Garmin Accounts from other people) I have not yet seen such a tool in the wild, only pics with range of units with identical serial number that I take for not faked judging the people who have published them. I am sure they use this to sell you "orignial European units" as the serial number should usually be enough to send the gps to German support (instead of having to send it to Garmin UK). Too bad that those groups not start hacking the firmware to offer some needed functions (e.g. reading tracks from SD, or optimising the routing algorithm for offroad navigation) but only focus on cracking for commercial success.
to sad they don't and we are offering so much for free with OSM