Security rule for Tax and and control Account

Dear Guru's

My client does not want the users to be able to code invoices in Payables directly to the VAT Account or the Liabilities account.

I set up a tax security rule to stoip the direct coding which works but then wont allow the tax calulation.

Also what about interfaced invoices?  Wilkl that security rule reject the tax lines?

Could someone please help me with this.

Thank you

S