Hi,

You might want to do this in the other way around, if the user inserts invalid characters , they should get a error saying that special characters are not allowed. The validation would require something like a regex that would only allow letters, spaces and numbers, if it makes sense to have them. 

In case you really want to remove those characters, you would need to use Replace function but you would probably end up building this, that doesn't look pretty. 

or you can try using Regex_Replace from Text extension