How do i find the GUID/a unique client hash not IP-address

hi, on a site i am currently making i have an open Comment Box so any one can leave a comment. i dont thin it will be a igh traffic site so there is no issue there.

however is doesnt take a genius to realise that some people will abuse, is there any way of taking a Unique Identifier from the client. so i can ban all input from a specific computer. i would use IP but most systems (at least in UK) have there IP refreshed with there isp.

many thanks, sorry if my questions sound dumb im new to asps.