Restricting Users through Validation rules

Restricting Users through Validation rules:

 Using Profiles and Userids We will restrict users in Salesforce.

IF( ISPICKVAL(<FIELD_NAME>, "<FIELD _VALUE>") && $User.Id = "<USERID_NUMBER>", TRUE, FALSE)

For example, this is the formula I used:

1.IF( ISPICKVAL(Type, "prospect") && $User.Id="005900000034iOC",TRUE,FALSE)

 2.IF(OR( ISPICKVAL(Type, "prospect"),ISPICKVAL(Type,"other")) && $User.Id="005900000034iOC",TRUE,FALSE)

3.AND(  OR (ISNEW(), ISCHANGED (Type)),  OR (ISPICKVAL(PRIORVALUE(Type), "Customer"), ISPICKVAL(Type,"Customer")),  OR ($User.Username <> "xxxx@nnnnn.com"),  OR ($User.Username <> "xxxx@nnnnn.com"))