In order to stop using the project in your project, you need to have an Administer Projectpermission assigned.
If you no longer need to use some project, you can easily remove it from your project usages.
Removal can be:
Complete, after which all references to the elements stored in the used project are eliminated.
Incomplete, after which all the references are preserved. Choose to remove the usage this way, when you need to replace the used project with an equivalent project that contains the same elements with slightly different content.
Before you attempt to stop using a project, make sure, other users currently are not creating usages to this project.