Firstly MAXL is NOT an option in PBCS.
Following steps can be taken to clear the Physical Data.
Login to the Calculation Manager and
Expand the Planning application
Actions-> Database Properties ->
Expand the planning application > Right Click the DB -> Clear -> (All Data, Upper Level Blocks , Non input blocks , Dynamic Blocks and Empty Blocks ).
This lets you type an MDX command and on completion, a message of successful clear is returned along with the number of cells cleared.
If run the same command again , get the double the number of cells cleared than last time. So it seems to be doing a logical clear and adding offset cells every time a partial clear is performed.
Per Development team, options are now available for Logical or Physical in partial clear for ASO on the Planning 188.8.131.52.608.2346 and Calculation Manager 184.108.40.206.608.011