did you ever think to look at one to see what's in there???
if you do an "lsof /vaw/log/warn" you should see what's writing to it, probably syslog, so you'd want to edit your /etc/syslog.conf (or equivalent) to change its behavior.
More fundamentally though, you probably want to pay attention to the log, as that's what it's there for, and address the root cause instead of masking it.
as for the old compressed ones, you can just delete them.