[efreak]

So make the sideload toggle like Android's bootloader--when you toggle it, wipe user data. Anyone who wants to side load will have to reinstall all their apps. There's other annoying things you can do too: require the user to wait for a delayed email; require unlocking through a website; require one if the above actions to be performed to toggle the state, and again for each side-load install (for example, require waiting for a 5 minute delayed prompt to turn on sideloading, and a 30-second delay for installing). If the side-loaded app requires sensitive permissions, only save them for a limited amount of time (ie, if it asks for access to your contacts, re-request every 2 weeks, or on every update). Pop up a long EULA-style description that needs to be scrolled to the bottom to continue.

There's dozens of ways to make doing things extremely inconvenient, to the point that people won't do them without a good reason, without being _overly_ bothersome to those that do want it. (Unless you're installing/updating dozens of side-loaded apps simultaneously, a delayed prompt shouldn't bother you much)