how about you just disallow all frontpage submissions until a certain karma level? make people comment before they're allowed to submit links or text content, since those are the easiest to game.
comments are easier for the community to moderate and its easier to pick out spammier ones, since they're clearly off topic or include random inappropriate links.
are they? we kind of tend to smash down comments that are even slightly off topic or non-contributory.
they might sneak in on the less popular submissions, but i kind of feel like it won't get by any more of a problem than it already is. if comments were more a effective way to spam, they'd already be a problem.
alternatives: disallow links in comments until a certain karma/age. disallow both comments and submissions (ha) until you've voted on enough statistically different things. make it a game by creating a new metric that people can be measured on based on the number of spammers they've sought out and identified via flagging, and create a new top list for it.
comments are easier for the community to moderate and its easier to pick out spammier ones, since they're clearly off topic or include random inappropriate links.