It would be nice with a plugin or some function you can use to delete posts in the database. It is difficult to dig in the database and know where and what to delete.
See my question at: Delete posts from database
Or perhaps a guide about how you can delete in the best way. It is difficult to know how the different tables are connected and referenced to each other. If we just delete some posts we might get broken references somewhere.
Some more technical documentation about the database(s) and instruction would be a good start. And hopefully easy for you to create.
Let me know if you need more information from me and I will explain more.