I don't see any easy way of accomplishing this.

If the tablespace has just the datafiles you want to remove and/or you do not care about the data in that tablespace you can drop the tablespace and recreate it.
(in such a situation, you may use "drop tablespace table_space including contents")

The otherway is to do an export of the objects from the tablespace where the datafile resides, drop the tablespace, recreate it accordingly, import the object/data.

Note:

By
"I found out that there are 2 unused datafiles (contains no file) on our database", do you mean that the files are not available on the OS filesystem but the entry is there in the DB?