What if you try deleting using a top-down approach (i.e. delete all asms then all parts)?
How much data and exactly what kind of data are you looking to delete? Just CAD Documents or CAD Docs, WTParts, and/or WTDocuments?
Is any of this data located in workspaces at present?
Shared to projects?
Added to baselines?
It would be a good idea to open a case with Technical Support to get further assistance with this.