No, of course you can't stop one user to login from multiple IP's (if i understood the question correctly). Many users use forums and one user can access them from multiple locations, or using a dial-up connection (that change the IP at every connection). What you want to do , it will ban most of your users .
If you do not trust that user that give login data to other persons, ban him, or do not give him administrative access to your forum (set him as a normal user).
Anyway, all forums have a backup copy on my server 10-15days old so.........just in case a trusted user hack your forum and delete posts, users, etc, because he is admin and he is upset (it happened before on other forums), we have all the time a back-up copy and we can restore your forum for you. 99% of the free forum hosts do not have this feature, extremely useful sometimes

Thanks !
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