At present this is not desireable or even possible.
Certainly, you can turn off the ConvertFontToSpan
Then, if you fill the editor with content that contains spans, they will stay as span elements. However, the browser rich edit engine in IE uses FONT tags when commonly used commands such as FontName, FontSize, ForeColor, BackColor are executed.
What happens next?
The browser engine is not able to optimize the nesting of tags, because it can only do so when FONT tags are used. As a result, a bad mess of deeply nested SPAN and FONT tags occurs - and the more the content is edited, the messier the resulting HTML is.
In case you wish to test this for yourself, you can use the following online demo:
To test you need to turn off the ConvertFontToSpan
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