You can drop a tablespace by redefining the objects to use different tablespaces before the import. You can then issue the imp command and specify IGNORE=y.
In many cases, you can drop a tablespace by doing a full database export, then creating a zero-block tablespace with the same name (before logging off) as the tablespace you want to drop. During import, with IGNORE=y, the relevant CREATE TABLESPACE statement will fail and prevent the creation of the unwanted tablespace.
All objects from that tablespace will be imported into their owner's default tablespace except for partitioned tables, type tables, and tables that contain LOB or VARRAY columns or index-only tables with overflow segments. Import cannot determine which tablespace caused the error. Instead, you must first create a table and then import the table again, specifying IGNORE=y.
Objects are not imported into the default tablespace if the tablespace does not exist, or you do not have the necessary quotas for your default tablespace.