How to remove words added by mistake from spelling check?

We can use CurrentValue to find the words that have been added to the dictionary for all notebooks:

CurrentValue[$FrontEnd, {SpellingDictionaries, "CorrectWords"}]

(* {"MyCompany", "AnotherCompany", "exx"} *)

We can update the dictionary by assigning a new list to this expression. This could be a completely new list or, as in the following example, we might delete just a single word:

CurrentValue[$FrontEnd, {SpellingDictionaries, "CorrectWords"}] =
  DeleteCases[CurrentValue[$FrontEnd, {SpellingDictionaries, "CorrectWords"}], "exx"]

(* {"MyCompany", "AnotherCompany"} *)

Words can also be added or removed from the dictionary associated with a particular notebook by replacing $FrontEnd in these expressions with EvaluationNotebook[] or any other notebook reference.

Reference: Add Words to the Spelling Dictionary.