I have already in a earlier blog post indicated how we can setup that a PMP goes only to certain people.
Now further, if you are using Objectives & Objectives library though it maybe possible to restrict whose scorecard will have valid objectives and whose doesnt. This in a sense can functionally restrict whose scorecard can get appraised by the supervisor and whose scorecard cannot be.
Objectives in the Objective library are linked to employees via eligibility profiles. Eligibility profiles can be defined based on many many parameters including date of hire, date of joining etc...now if you setup such a eligibility profile say only those whose 'length of service' should be minimum 6 months atleast and link it with an objective then a PMP which includes this objective will have scorecards with objectives only if the employee has been in the org for 6 months atleast.