If you search the mods forum you will find some discussions regarding blocking votes (ips). For the most part, you can't really stop others from voting a site down anymore than you can stop a site from voting a site up. Links does block multiple votes within a certain timeframe though. You can increase that timeframe to make it more difficult for someone to vote frequently. To do this, open rate.cgi and find:

(($time - $old_time) > 43200) and (@IP = ());

Increase that number to make the vote block last longer.

If worse comes to worse, and you suspect unfair manipulation of the voting process, you can always reset any given link to whatever rating and number of votes you want with the admin.