Hello.

I need to disable  [ spoiler ] bb-tag at all, because there is no adequate support of this tag in visual editor and every time when users uses it, they accidentally breaks layout of their content via this "feature" and it takes a time for our moderators fix broken posts.

Could you direct me to a proper way to disable it?

You can remove the spoiler button in ACP -> Design (pencil icon) -> Editor -> Toolbars. Just deactivate the button via drag and drop  there.

Unfortunately I don't know how to disable the BBcode at all. But above action should be the first step. Most users don't use what they can't see  

Yes, i know it, but my question was only about bb-code. I already don't have a button for spoilers in editor on my forums, but users has their own old habits to type bb-tags by hand and it worked well for them in 3.* but in 4.x it's a big pain so the best solution for now is just disable them at all. So i hope someone can show a proper way how to implement this in IPS.

Pretty sure that is hardcoded in the core files and can’t be deactivated easily. Certainly not with settings.