How do I stop someone from using or voting from my IP address?

You're making one huge assumption that's likely not correct. It's
very unlikely that there's fraud here at all.

There are a couple of very valid ways that the same IP addresses might
appear to be in use by several different computers. If the voting software
using only your IP address to check for voting abuse, then it's not doing as
good a job as it should be.

Two scenarios come to mind immediately: routers and bad IP detection.

As I've discussed before, a router allows several computers to share a
single internet connection. One side effect of doing so is that all the
computers that are sharing that connection appear on the internet as having the
same IP address. The internet IP address actually belongs to the router, and it
manages the routing the appropriate data to and from the appropriate computer
behind it.

If you're in a home or small business that uses a router to share an
internet connection, then any of the other computers could have voted and
appeared as having come from the same IP address your computer would.

If you're connecting from a larger facility, say a school or corporation,
the same issue can arise. Many facilities connect their internal network to the
internet through a set of routers. A company of several thousand might appear
as only a handful of IP addresses on the internet. If someone else within the
company voted, it's quite possible that you might appear as the same IP address
when you attempt to vote later.

And then there's AOL.

When last I checked, I believe AOL puts all of its subscribers
behind the equivalent of a collection of routers. Therefore when you as an AOL
user connect to a web site you may be using the same IP address as some other
AOL subscriber who came before you.

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There's another problem that could be at play here as well.

If you're behind a router, your IP address is an IP address on your local
network, not the internet. Most consumer routers use the 192.168.1.x range. In
fact, most will start assigning IP addresses at 192.168.1.1, then 192.168.1.2,
then 3 and so on.

As a result there are thousands of machines out there at IP address
192.168.1.1 on their local network.

If the voting abuse software attempts to use the IP address of the
machine rather than the IP address of the machine's connection to the
web site, then it'll just be wrong. The first person with an IP of 192.168.1.1 might
be able to vote, but all that follow would be seen as coming from the same
address.

It's very unlikely, but it is one additional way that the voting abuse
detection scheme might fail.

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Using just the IP address to counter voter fraud and other types of abuse is
common because it's easy. As you can see, that often has the unintended
side-effect of blocking more than just a single abuser. Unfortunately blocking
an individual computer for these types of accesses is difficult - at least
doing so in a way that isn't trivial to circumvent is difficult. The most
common alternate approach is to require registration and login to vote, which
naturally reduces the number of voters as people are reluctant to jump through
the additional hoops.