You could sanitize it by using a regex. You would have to know what you are looking for, and what you want to replace it with.
If you remove them, you will probably be removing characters that are wanted! This is the lower-case 'a' with a "ring" above it used in Scandinavian languages. But, ISO-8859-1 character set.
But, if you prefer, you can replace them with "bork, bork!", a blank space, nothing, or anything you would like. But that is probably not what is wanted! ;)
I usually don't like to link to W3Fools. But in this case, they have a nice chart, with a "Try It" for every character.
A more modern approach is to have the server sent UTF-8 characters, and use a meta to tell the browser to expect UTF-8. The author would just write the text with their text editor set for UTF-8.
You didn't show a complete example, so I can't guess at your <meta> tags!