If your testers are all part of a User Group you have defined, you could limit access to all of the modules so that they could not change anything in those module.
This process is defined in Chapter 15 of the HP ALM Administrator Guide, more specifically under the sections of ''Setting User Group Permissions'' and ''Customizing Module Access for User Groups''
However, if all of the users you are trying to limit access to the project are part of a standard user group, this cannot be done.
Also, if the list of user you want to limit access to a project is not too long, you could remove them from the Project Users list as described in Chapter 14, ''Removing a User from a Project''. Of course, they would then all have to be added back once the list was updated.
Unfortunately, there is no other way to limit access to a project for a group of users.