Yes we can decrease the size of the datafile to the place where the extents are deallocated in to it by resizing the datafiles to something smaller than current. If the data in the datafile is scattered than reorg the object in the tablespace to deallocate the extent and decrease the filesize

We can reduce the size of the datafile till the high watermark of the datafile. IF we have to bring down the high watermark then we have to check the extents in the file and reorganize objects as required.