I use an iOS app that autostarts whenever I reboot the phone. How can I disable that ?
In general, an iOS app is stateless.
Whether it's running or not, only the core iOS scheduler knows if a process is loaded into RAM and running or paused, suspended, or "frozen" to storage.
If you are referring to the list of icons that show in the multitasking bar - that too is stateless. All the programs that have been run show up in that list - even if you haven't run them yet on this boot. In fact, that list is backed up and restored onto a new device, so you could have an app appear in the multitasking bar that has never run ever on that device.