The most access that I know of is disabling or enabling spellchecking on a field: Inline Disabling of Firefox Spellcheck?
Browser's don't provide access to their built-in, proprietary spell checker APIs. I'm quite certain there's no x-plat way to do this, let alone a way to do it individually for each browser.
I think the most they'll allow is what Bobo said; you can enable/disable it for textboxes, but I don't think they allow any further control than that.
If you want control over spell checking, you could do it yourself with AJAX.
(And, incidentally, ieSpell is a free spell checker plugin for IE that is pretty easy for users to install on their own)
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