Billing docs are released with billing blocks

Hi gurus

I am a PM/CS consultant... i am configuring a CS scenario where, i create a contract for a Material VA41.

the contract is for one year, i am using periodic billing, i am not using milestone.

The billing dates scheduled for every start of the month. I have maintained billing block in billing plan. Billing plan is got for one contract period.

Once i done with the contract, i go to create billing documents in VF01

The problem comes here... All billing docs are released to accounting... i have used billing block.. but i am not sure wat is the problem

I need to bill only one doc in a month...

Kindly help me out...

- Pithan