Hello there 
I’ll go through each banner starting from the top first and explain what to do, I hope that will make it easy to read through and make the changes.
For the first banner on the notifications, clicking the X button at the far right will disable it.
For the second banner on bootstrap mode, setting 0 for the bootstrap mode min users site setting will disable it. Do note that it is a feature to help you help your users onboard faster in a new community without much posts or activity. So you can leave bootstrap mode on by not changing the setting to 0 to disable, but hide the banner with CSS.
For the third banner, only staff can see it, you can hide that one with CSS, not sure if there is a site setting for it.
To hide banners with CSS or make other CSS changes, this howto can help How to make CSS changes on your site