The solution is pretty simple. Asphyxiate him. Oxygen deprivation by surrounding him with burning fire. Create an enclosed space and fill it with water. Bury him alive. There are many ways.
WMG here.
Strictly speaking in terms of power, Bellzebub might be comparable to Azusa. But she is a demon, and a citizen of demon empire, so it would only make sense if some specific spell or restraint are there which only work against demons to ensure the demon lord peaceful rule. Azusa...
@Blackstone
All you've heard of them are all true. Never underestimate SEA traffic, almost all drivers are basically daredevil stuntdrivers, but even they never come within 3 meters from a momma riding her scooter.
There are no moral issues if you don't follow what is considered moral.
Fun fact, enlightenment also release you from such petty concept such as morality