Hi Manish,
Blocking Cost Centre against postings with Indicators in Control Tab blocks only inputs into CC.
The below is taken from help to field "Actual Secondary costs":
Use
If the indicator is active, you cannot post secondary costs to the cost center from now on.
This indicatory normally locks the cost center only against actual debits of secondary costs. However, actual credits to the cost center are possible.
So in your case system behaves as designed as you are allocating activities from CC, so you are crediting it.
If you want to block CC against all postings (both debit and credit) please change its validity dates.
Regards,
Szymon