Sorry for the delay, Hassan. If you set the Visibility for the Newsletter subscription web part using the following on-line marketing macro, you can hide it to users who are already subscribed:
{% !OnlineMarketingContext.CurrentContact.SubscribedToNewsletter(ToGuid("FBC356CE-D9D1-4F6C-9AB8-30B78C2DD4C2")) |(identity)GlobalAdministrator%}
The GUID is the GUID for the individual newsletter. The documentation suggests that you can use the newsletter name, though I could not get that to work.
Doing this you could place another web pert in the same zone with the opposite macro results **_or_** use a content personalization variant using the same macro if you want to give them access to another variation of the newsletter sign-up.