yes, the fastest and easiest way to remove default gateway from ODM is
smitty route

or do the following steps

1) Check the default gateway
# netstat -Crn

2) go to smitty tcpip
--> further configuration --> static routes --> Flush route (last option)
there are four options: make all the options yes yes and hit enter

3) go back and check if it has been flushed or not
netstat -Crn