Theres actually a setting in the cp to disallow guests viewing profiles; that should be a sufficient way of doing that in general.

But, if you dont' want that on, then:
Code
Disallow: /forums/ubbthreads.php?ubb=showprofile
Disallow: /forums/ubbthreads.php/ubb/showprofile

If you didn't want them spidering the userlist, then this as well:
Code
Disallow: /forums/ubbthreads.php?ubb=showmembers
Disallow: /forums/ubbthreads.php/ubb/showmembers

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