This article explains how to delete process chain logs in BW 3.X environments. Process chain logs are stored in RSPCLOGCHAIN and RSPCPROCESSLOG tables.
In BW 7.0 there is a standard program RSPC_LOG_DELETE to delete these logs. In 3.5 you need to build a custom program to achieve this. This document details on how to achive this.
Please have a look at note - 1275318