Mechanical soldiers and cursed children are shown to have superhuman strenght. A cursed children of one kick can blow a Gastrea and a mechanical soldier (Rentaro) of one kick can destroy several floors. How is that we see then a homeless cursed children being trapped by 2 or 3 police officers or Rentaro being held by a couple of humans? Can their strenght can be deactivated somehow or something?
I'll answer this in two parts, the first part covering mechanical soldiers and the second part covering cursed children.
No it is not possible to deactivate/remove a mechanical soldier strength remotely. This is easy to see from the fact that the only way to defeat Kagetane (who is also a mechanical soldier) or Tina was to beat them outright. In both cases their identity were known but there was nothing that could be done to stop them beside facing them on the battlefield.
It has to be mentioned that it might not be impossible that some mechanical soldiers might have a kill switch built in (though I have yet to find any mention of this in the light novels) but this is unlikely due to the same reasons that it is stupid to provide a back-door to a system you built. If you can get in sooner or later someone else will also figure out how to get in. Also, if you ever get captured it is very likely that the people who caught you will force you to disable your soldier which makes the chance of you being rescued unlikely.
The reason the police were able to restrain Rentaro is simply because he was holding back. Similar to our world it puts you in a very bad place when you beat up police officers who are restraining you. Rentaro (and by extension the Tendo Civil Security Agency that employs him) would not be able to continue doing business if he beats up police officers even if he was in the right.
Cursed Children can be harmed/killed with Varanium weapons (see this question). While it might not be easy to hit them due to their abilities once they are hit they are as vulnerable as any human. Additionally Initiators are trained fighter whereas homeless Cursed Children are not. Then there is also the fact that the Cursed Children are already hated and feared (which is what
leads to the bombing of the bombing of the outer district school)
so it does them no good to resist. They will most likely just be "put down" as most people do not look at them as human. Additionally it will simply incite further violence and hatred against their kind.
The following extract indicates that Varanium weapons do exist with the explicit goal of wounding and killing Cursed Children as well as the general response of Cursed Children when faced with violence
“Damn it, what a disgusting brat!” Facing front, Rentaro saw that the oldest man in front of him was holding a Varanium switchblade dripping with blood, his face twisted with hatred. It was a thin knife with a blade about twelve centimeters long; it was too short to fight off a Gastrea, so it was hard to think of what it could be used for other than to deliver a fatal wound to one of the Cursed Children. The beggar girl shook her head, still bent over on the ground. “I’m sorry, Mr. Civil Officer. Even though I promised…I deserve what I got, but I, I just…”
Kanzaki, Shiden. Black Bullet, Vol. 3 (light novel): The Destruction of the World by Fire (pp. 163-164). Yen Press. Kindle Edition.