"Mailing" tab won't disappear after disabling CiviMail component
This appears to be a bug - I was able to reproduce it as well. Normally disabling a component should hide the menu items associated with it.
If this is important to you, you could try to figure out what's going wrong and submit a pull-request with the code to fix it, or else provide funding to fix it in the paid issue queue. Unfortunately the core team doesn't have capacity to fix non-critical bugs unless someone provides code or funding.