Disallow HTML in comments
The Allowed Formats projects allows you to control the available text formats per field, so you can limit it to plain text. Better Formats is a similar module.

You could do the same with a form alter and set #allowed_formats on the form element. Or you could remove the default comment body field and replace it with a plain text field. Although there might be some code relying on the name/existence of the default body field.