alter database datafile ... offline drop

Dropping datafiles
Datafiles were not designed to be dropped. (See also Metalink note 111316.1) This is true even though there is alter database datafile offline drop.
However, if the datafile is the only datafile in its tablespace, it can be removed together with the tablespace:
DROP TABLESPACE ts_data INCLUDING CONTENTS;
With Oracle 10g Release 2, it's finally possible to drop a datafile as long as
it's not the only datafile within the tablespace, and
the tablespace is only and in read-write status
:
alter tablespace ts_something drop datafile '/path/to/datafile.dbf'

Fuzzy datafiles
A datafile that contains a block whose SCN is more recent than the SCN of its header is called a fuzzy datafile.