You want to stop it from being rebooted or it's rebooting by itself? What tablet is it, make, model and Android version? ...possibility of known stability issues with certain devices and firmware.
If it's rebooting by itself, unexpectedly, then there's likely a stability issue with it, possibly buggy firmware and it needs to be updated. And if you want to stop it from being rebooted, that might be difficult, because usually they can be rebooted by holding down certain buttons which causes them to restart, like power, which is hardware.