Why does your client want to delete all materials, even ones with transactions?
There is nothing such as going back to a clean slate in SAP. In case your client insists, this is procedure:
1) For the materials with transactions, you will have to mark the documents for deletion and then run the corresponding archiving object in SARA.
2) Once all the related transactions have been archived, use MM70 and MM71 to archive the materials.
Please note that this archiving can be easily traced and will not be easy to convince any SAP auditor. Proceed with caution!