Hi Hasan,
Please can you confirm if it's ok to delete a posted journal that I do not want at all. If true, can you provide me a small script to do that please.

Hello,

You don't need to delete the unwanted POSTED journal.
.
Simply reverse the journal in the same period and POST it...
.
That's it
Charlie :)

You should not delete the posted journal. Reverse the journal instead.
Posted journals have affect the balances in GL_BALANCES table. Deleting journals will throw the balances off.
Whether posted or not posted, you should not be deleting journals from the back end. Keep the system clean by following right practices, otherwise you may have data corruption no matter how careful you are.

Thanks for quick come back. Yes I agree with you, but where the data is not meant to be seen by other people what is the best option. Reversing is not because the data will still be there. Any other option?

Once you post a journal then the audit trail has to be maintained (from auditing perspective it is required) All users having access to that company (within the ledger) should be able to see the journal details. Same applies to the reversal. Bottom line is that you cannot (and you should not) be able to delete a posted journal. Oracle does not allow you to do that.
Having said that, you can control which users can see which ledgers using Data Access Set security and also you can control which users could see which companies within a ledger using either Data Access Set or Security Rules. If you wish to hide details then you can apply one of these methods.
If you still wish to hide details then something is wrong elsewhere. You should re-evaluate how can protect your system from unwanted journals. Or may be you need to re-evaluate who should have access to the system. May be business procedures and Oracle security needs to be revisited at your end.