I can not stand books that are not fully justified and hyphenated, but if you prefer to stop hyphenation the following tip might help.
Mobile ADE libraries in Nook STR(1.0.1 update) have a bug that shuts down the hyphenation engine until the device is turned off and on again. If the Nook Color uses the same library code, maybe you can (ab)use this bug to turn off hyphenation.
Here is how:
1. Prepare a dummy EPUB (or acquire a free one from gutenberg, feedbooks etc.)
2. Extract OPF file from it.
3. Change the language to "UND".
4. Put it back into the EPUB overwriting original one.
When you open the resulting book in Nook STR, the hyphenation engine seems to be getting shut down and all your other books are rendered without hyphens as well. This behavior persists through sleep. The engine starts running properly again after the device is turned off and on.
PS. Since this is a bug abuse, do not expect it to work indefinitely. A patch in a future update might fix this behavior.