The DocumentSpellChecker.Remove(srting str) method removes the word from a Custom Dictionary associated with Invariant Culture. The word "repot" is in the main dictionary - RadEn_UsDictionary which you can't edit with code.
What you can do is to build your version of RadEn_UsDictionary, which does not include "repot". In order to do that, you should remove the reference to Telerik.Windows.Documents.Proofing.Dictionaries.En-US. Then, modify the en-US.tdf file using a text editor, locate the word and delete it, then save the file. You can download the en-US.tdf from the source of RadControls for Silverlight or from this forum post. Create a custom dictionary that loads the en-US.tdf file as described here.
Please try that and get back to us if you need further assistance.
the Telerik team
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