In the Projects section, you can disable the creation of new projects. Once active, users will only be able to track time to existing projects and cannot create new ones.
This setting can only be managed by the Company Owner or Company Administrators.
To enable the option to create new projects, go to Settings → Account → General → Employee permissions and make sure that the Enable new project creation button is turned on.
Instead of disabling this option Account-wide, you can restrict specific users from creating new projects instead. In this instance, go to Settings ➡ Team Members ➡ Edit, and disable Project creation from there.