There's no way to prevent tables from being loaded into memory, except dropping them.
With SP6 you don't have access to the LOAD statistics, so it can be a little bit difficult to find out what causes the table to be loaded to memory.
However, why do you feel you need to manually unload it?
If the table is not actually accessed at all (btw: what does make you so sure it's not accessed?) then SAP HANA will remove it from memory, as soon as more free memory is actually required.
- Lars