How to restrict user to post in GL a/cs thru validation rule ?

Hi Experts,

I want to restrict an user to post transactions for tds related gl accounts.is that possible through Validation rules.

Actually i created a step and under that i configured prequisite,check and message.

In "prerequisite" Company code = xxxx and user name = fffffff

in "check i configured, all restricted GL accounts

In "message" -

GL account restricted to post by user fffffff

But still the problem is not yet solved as the user is able to post transaction in that GL accounts.

Plz guide.

Sumeya offrin