To implement the above, please follow these steps:
1) Create a toggle field on the 'Request' entity and name it something like 'Block Discussions';
2) Create a Validation Rule on the 'Discussion Post' entity that would run only when a record is created with Evaluation Criteria: Catch($Container.C_blockdiscussions, FALSE)
3) Create a Workflow Rule that would turn on the 'Block Discussions' toggle whenever a Request is converted into a Project.
Please let me know if this helps.