It's looking quite likely it was a spammer trying to mask their IP with a proxy:
http://www.stopforumspam.com/ipcheck/5.9.158.75
Thats not to say all proxies will mean "spam", so use with caution. Some people don't even know they are using a proxy.
Some ISPs in certain countries will even use transparent proxies, but for the uk it's not that common
There's a list of options for which proxies types you can allow through / prevent. I would keep an eye on it
These are the transparent proxies you can turn on / off
- Block Transparent Proxy Header: HTTP_VIA
- Block Transparent Proxy Header: HTTP_X_FORWARDED_FOR
- Block Transparent Proxy Header: HTTP_FORWARDED_FOR
- Block Transparent Proxy Header: HTTP_X_FORWARDED
- Block Transparent Proxy Header: HTTP_FORWARDED
- Block Transparent Proxy Header: HTTP_CLIENT_IP
- Block Transparent Proxy Header: HTTP_FORWARDED_FOR_IP
- Block Transparent Proxy Header: VIA
- Block Transparent Proxy Header: X_FORWARDED_FOR
- Block Transparent Proxy Header: FORWARDED_FOR
- Block Transparent Proxy Header: X_FORWARDED
- Block Transparent Proxy Header: FORWARDED
- Block Transparent Proxy Header: CLIENT_IP
- Block Transparent Proxy Header: FORWARDED_FOR_IP
- Block Transparent Proxy Header: HTTP_PROXY_CONNECTION