Board or external, the voting will be always affected by manipulation. Unfortunately this is something that can't be avoided.
Valent - It can't be avoided, but it can be made harder At least having an email registration, password, email confirmation, etc., are additional steps that may dissuade the easiest cheating.
This way, nothing stops a person with 2.000 friends on Facebook to post a link and say "hey, vote option X please". It's just two clicks anyway.