Hi Joanna,
When you assign a task to a user, Workfront typically automatically grants them Contribute access to allow them to interact with it, log time etc. But if you assigned a user a task, but then immediately set their sharing access on that specific task to 'View', It would accomplish preventing them from logging time, though still appear on their timesheet. See screenshot sample below, the task/project are visible but can't be touched.
Extend that a step further and revoke even 'View' access, and the task/project will disappear from their view entirely.
If you wanted this functionality to happen with a few less steps, you could start your project used for this planning purpose by tweaking the security settings under Access to say 'when someone is assigned a task, give them View access to the task' only. Contribute is a normal default there.