You can use: alter table xxx MOVE to move each table (1 at a time)
to another tablespace ... Coalesce the free extents in the tablespace.
and move back ... you need to map the blocks in the tablespace in
question to find out what objects need to be moved and removed...
Note: this will invalidate indexes associated with the tables that are moved... You will have to rebuild the indexes.

HTH
Gregg