It is not clear from your post whether you are referring to the AJAX spellchecking in RadEditor for ASP.NET AJAX, or whether you have your own editor/textbox that uses the independent RadSpell control.
In either case, it should be possible to achieve this functionality and "strip" portions of the content that you wish to not be checked. Please let us know whether in your scenario you use RadEditor so that we know how to approach the issue and help you further
Looking forward to hearing from you,
the Telerik team