No, they stay there. Once they expire they really mean nothing expect as a history for us to know who's been a problem (and what people have been warned for).
Only speaking for myself here, but If i'm dealing with an issue with a member, and their last infraction/warning is a year old, I tend to just write it off as "must have been a bad dayt" and just ignore it. After all, everyone has bad days and that could have been one of them.
"as a fanboy i refuse to admit it and will pull countless things out of my butt to disprove it"
Team Thelegorm! Total Kills: 21 (i iz in uor profile, editsing your sigz)