You can also use the fwtmp command to convert wtmp to ascii, edit down, and use fwtmp to convert it back. Check the man page for location and usage of fwtmp.
If you grok C, the utmp entry format is documented in /usr/include/utmp.h. Document carefully, though. The format can an has changed between AIX versions.
Rod Knowlton
IBM Certified Advanced Technical Expert pSeries and AIX 5L
CompTIA Linux+
CompTIA Security+