You're more than welcome to suggest this but the amount of users who will show up will be huge for even a small-medium sized community if there were a user agent list and would not fit the needs here either due to just sheer numbers.
Every system to track a user is not without flaws. Typically, the most accurate are cookie or IP address based but those can be bypassed by those who know what they're doing. When dealing with troublesome members who are also tech savvy, the only way to thwart their attempts is going to be human eyes reviewing things.