The markup code “HTML” inserts can be caused by a number of factors, ms word can markup text with html and some browsers do the same to preserve formatting.
You are using a WYSIWYG editor on this website that requires html code markup the text so it displays properly.
The solution for the frontend of you website is to have your web devs apply a code removal/cleanup plugin to the text editor and configure it properly.
For the backend (server side) you will also need to strip unwanted html markup using kess or htmlawed.
You can correctly assume that most users will be clueless to these technicalities and won’t even realise they are inserting extra markup.
Thats what WYSIWYG editors are for – What you see is what you get. This requires markup in the form of html code to display text formatting properly in the web browser.