Lock table only allow INSERT

Hi, 
recently, i was assigned as a new dba for our system. I found that my
statistic  keep change from time to table. To look for the cause, i
wanna to lock a table, means only allow user to insert. Others than
that, noway.

Can i use the Lock table command? or others comand?

p/s: we are using 9i client and 6i developer
regards,
Lee Cho