From man ifconfig :

Tip: Parameters that you set using the ifconfig command are lost the next time you restart your system. Use the chdev command to change the ODM database for each interface to make parameter changes permanent. Use the lsdev -E -l [interface] command to view the interface attributes and use the chdev -l [interface] -a [attribute=value] command to change the attribute. For example:
lsattr -E -l en0
chdev -l en0 -a tcp_sendspace=65536


so :

lsattr -El inet0
chdev -l inet0 -a delroute=[]


btw, the easiest way is smitty route