[galaxy-user] Can not delete data from my local galaxy

Hello, I am using local galaxy server. I have disc storage problem and I noticed that galaxy is utilizing most of the space. I deleted old projects permanently, but they are appearing again in the saved history with one message which says "datasets were not removed from disk because that feature is not enabled in this Galaxy instance". Any suggestions how to enable this feature or delete dataset permanently from history. Galaxy installed on ubuntu 12.04 LTS, 3.5.0-44-generic, 64bit. Best regards Nishant Thakur