The command is unassign(varname).

The memory allocation shown in the bottom r.h.s. corner of the GUI suggests no memory has been freed up; watching the value of kernelopts gcbytesavail and gcbytesreturned suggest garbage is hanging around waiting to be collected, and that this is being done on a call to gc(), but kernelopts memusage suggests the memory is being freed and garbage collected right away.

I'd be inclined to believe kernelopts before the GUI display, BUT given that Maple is crashing for me inspite of my use of unassign, I could be completely wrong in doing so!