Forms :: Clear Block B Once Click FIND Button

I am working on form builder 6i.I have FIND widnows and MAIN window.

FIND Window: contains field1 and field2, FIND Button.

block A: Multi record data block on table t1. It contains field1,field2,field3.
block B: non datablock, it contains field4, field5.

If i enter something and click FIND button, it will populate data in Block A.If i put the cursor in any record in block A, and after that put the cursor in Block B, it will populate block B related to Block A record.For this, In Block A--> post record(block level) trigger, i am capturing values in variables.

:GLOBAL.f1:=:BlockA.field1
:GLOBAL.f2:=:BlockB.field2

In Block B--> When-New-Block-instance(Block Level)

select field3, field 4 from t2 where t1=:global.f1 and t2=:global.f2;

and populating data in block B.It's working fine.

REQUIREMENT: If i click the FIND second time, in Block B--> it's retaining previous value. I want to clear the block B once i click the FIND button. I tried to write in POST-BLOCK (Block B-->block level)

CLEAR_BLOCK('NO_VALIDATE');

But it's not working.In which trigger i have to write code to clear block.