To prevent web user from using HTML Tags in comments or block particular HTML elements, follow these steps:
1. In the folder wp-includes find the file kses.php and open it.
2. Find the text $allowedtags. It is followed by a list of the HTML elements allowed in comments.
3. To prevent a particular HTML element from being used, comment out the line for that element. To comment out a line, type // at the start of the line (some elements will probably be commented out already).
4. If you never want any HTML in comments at all, you can delete the whole function — from if (!CUSTOM_TAGS) to the next }.