Permissions for each Mashup will need to be restricted to control Visibility and the ability to edit/execute that mashup (Edit Mashup > Info > Permissions). There is no automated ability to assign a user to a single mashup and nothing more -- permissions for all Mashups and things in the system will need to be configured appropriately to restrict access.
Muhammad, indeed as Tudor mentioned you will need to restrict the access rights for users. Consider grouping the users and assigning them rights collectively, this way you can reduce the effort on individual user configuration for Mashups or even other entities. Would strongly recommend you to go through the User Groups documentation and of course all the security setup around it in the documentation.
You can also consider setting up the landing pages for all the users by setting specific Home Mashups.
Hope this will help.