You should use the DROP DATAFILE and DROP TEMPFILE clauses of the ALTER TABLESPACE command to drop a single datafile or tempfile.
Here are some examples.
ALTER TABLESPACE example1
DROP DATAFILE '+DGROUP1/example1.dbf';
This other example drops a tempfile.
ALTER TABLESPACE temp
DROP TEMPFILE '/u02/oracle/data/temp03.dbf';
The database must be open and the datafile should be empty.
You cannot drop datafiles in read-only tablespaces and in the SYSTEM tablespace.