I thought I would post this solution, in case it may be of help to others.

A colleague today asked for help with a couple of client documents that would not spellcheck completely and that, of their own accord, kept checking the "Do not check spelling..." box in the Review/Language/Set proofing language dialogue, no matter how many times it was unchecked and the document saved.

Some detective work suggested to me that one particular style was probably the culprit, but attacking that alone did not resolve the issue. This was one of those documents in which Normal, Table number, and various other styles had each been locally formatted in just about every conceivable way, and it appeared that somehow the problem had gotten bound up with a character overlay as well. (Stranger things have happened in Word...).

I finally looked for a global replace option and quickly found that to spell-check or not to spell-check can be employed in Find and Replace with "No language", that is, regardless of language; "No language" is an item at the top of the list of available languages. Therefore I used Find to look for the "Do not check..." enabled with "No language," and Replace with "Do not check..." disabled, similarly. I selected Replace all. Word made a number of replacements, and the document is back to it's miserably style-mangled self again, this time with spellcheck operating.

(No, I am certainly not suggesting that documents be left like this; proper use of styles, fresh documents, etc., are the standard and my professional creed. This solution obtained in a time-sensitive bind for limited purposes. It may come in handy now and again.)