There isn't any way to wrap the log from SQL (at least in ASE - don't know about ASA).
One way to do this under Unix is to:
cp logfile logfile.save
cat /dev/null > logfile
This works reasonably well - just do it at a time when activity on the server is low as there is a small window (between the end of the cp and the cat) where an error message written to the log could be lost.