[KevinH]

Given hunspell is supported in Sigil plugins, you could do a file by file spellcheck in a specifically designed Sigil plugin if you are familiar with python programming.

Alternatively, you could use find and replace to add or change the language attribute in the html tag in all files you wanted to exclude from spellchecking to be some unsupported language code. Words without a dictionary in that language are considered correct (not marked as errors) in the Spellcheck dialog.

Updating that a page at a time should do what you want.