Well they aren't trying to login, they're simply hitting the login page through crawling. You could setup a robots.txt directive (for the bots that respect it) to not crawl some UBB.threads URLs such as:
Code
# Set any global page blocks
User-agent: *
crawl-delay: 10
Disallow: /forum/admin/
Disallow: /forum/templates/
Disallow: /forum/ubbthreads.php?ubb=calendar
Disallow: /forum/ubbthreads.php/ubb/calendar/
Disallow: /forum/ubbthreads.php/ubb/calendar.html
Disallow: /forum/ubbthreads.php/ubb/calendar/month/*/year/*.html
Disallow: /forum/ubbthreads.php?ubb=dosearch/
Disallow: /forum/ubbthreads.php/ubb/dosearch/
Disallow: /forum/ubbthreads.php?ubb=editbasic
Disallow: /forum/ubbthreads.php/ubb/editbasic/
Disallow: /forum/ubbthreads.php?ubb=login
Disallow: /forum/ubbthreads.php/ubb/login/
Disallow: /forum/ubbthreads.php/ubb/login.html
Disallow: /forum/ubbthreads.php?ubb=markallread
Disallow: /forum/ubbthreads.php/ubb/markallread/
Disallow: /forum/ubbthreads.php/ubb/markallread.html
Disallow: /forum/ubbthreads.php?ubb=mycookies
Disallow: /forum/ubbthreads.php/ubb/mycookies/
Disallow: /forum/ubbthreads.php/ubb/mycookies.html
Disallow: /forum/ubbthreads.php?ubb=myhome
Disallow: /forum/ubbthreads.php/ubb/myhome/
Disallow: /forum/ubbthreads.php?ubb=newuser
Disallow: /forum/ubbthreads.php/ubb/newuser/
Disallow: /forum/ubbthreads.php/ubb/newuser.html
Disallow: /forum/ubbthreads.php?ubb=online
Disallow: /forum/ubbthreads.php/ubb/online
Disallow: /forum/ubbthreads.php/ubb/online.html
Disallow: /forum/ubbthreads.php/ubb/printthread/*
Disallow: /forum/ubbthreads.php?ubb=search
Disallow: /forum/ubbthreads.php/ubb/search
Disallow: /forum/ubbthreads.php/ubb/search.html
Disallow: /forum/ubbthreads.php?ubb=showday
Disallow: /forum/ubbthreads.php/ubb/showday/
Disallow: /forum/ubbthreads.php/ubb/showday.html
Disallow: /forum/ubbthreads.php/ubb/showday/day/*/month/*/year/*.html
Disallow: /forum/ubbthreads.php?ubb=viewmessages
Disallow: /forum/ubbthreads.php/ubb/viewmessages/

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