We came across the potential for abuse with the 'rate a site' system with the phpLynx script, its not difficult to impose a one click per IP to prevent abuse, there's other variations of this as well, such as time restrictions (number of times a person can click the 'rate me' or top sites, within a given time frame. i.e, no repeat visit from same person within 24 hour period kind of thing. As much as there are people who abuse these systems there's always ways to beat that by thinking ahead of them.
There's already a setting in the script I use to limit the hits time range from a single IP, but it's still abused. I'll also bet that if the top hits page was 'nofollowed' by default, this problem wouldn't exist. I think the primary attraction is that the PageRank of the top hits page is higher than some category pages. Some have set the number of listings on the top hits page to 50 or 100 to discourage them, but it seems to have little effect. It's encouraging to know that some have reset the 'hits' of those that may be abusing the feature, but I doubt this is the answer. I've removed it from the navigation as I just don't see it as useful.