Illegal characters in a RSS feed

I’ve carefully searched but have found no HTMLTidy plugin that works with current versions of WordPress.

This is important because I have users of my WordPress sites that are nontechnical and they just cut and paste nonvalid and control characters into their posts higgledy-piggledy. These then infect the RSS feeds.

This is a problem because those sites use an e-mail broadcaster (MailChimp) that scrapes the site RSS feeds to generate e-mail for customer lists. The illegal characters cause the mailbot to barf and reject the feeds and fail to send out updated news. This has continued to be a problem.

I have searched for WordPress plugins to automatically clean user input before during and after publishing. Ideally I’d like something like HTMLTidy to clean user input before updating or generating the RSS feed.

I don’t merely want a validator because I can’t trust my users to use that or to understand what the errors mean. I want something to actively clean up all the text and markup before RSS updates are generated. Perhaps a button I can press to do this to all data periodically?

Are there any currently maintained and supported plugin modules that do that?