https://www.yoursite.com/impex/tools/cleaner.php
...using your forum URL of course.
ImpEx parses out as much HTML and incorrect BB code as it can find, though there can sometimes be unexpected HTML or codes in the posts.
To remove this you will need to use cleaner.php, the basic principle is that is matches one string and replaces it for another, or a blank to just delete the original string.
Once cleaner has been run successfully you must rebuild the Rebuild Post Cache in the AdminCP > Maintenance > Update Counters
The file itself has many comments and instructions though as an example, if I have the post :
Hi there, I was looking though the web and I found this site and I think it <B>r0x0rs</B> , its about pirates it must be good as they are the best.
<a href="https://www.example.com/badpage.html">advert</a>
There are three errors there, the <B> tags should be BB code, we want to remove the HTML completely and pirates should be ninjas, obviously.
In the script there is this code :
$replacer = array(
"<img>" => "[img]",
"" => "",
"" => "",
"" => "",
"" => ""
);
$replacer = array(
"<B>" => "[B]",
"pirates" => "ninjas",
'<a href="https://www.example.com/badpage.html">advert</a>' => ""
);