Try using the command
alter table table_name deallocate unused;

to return any unused space that is allocated to the table. Then
try
alter database datafile 'datafile_name' resize nnnn;

to shrink the datafile to the desired size. This may not succeed because some of the data may be distributed all over the datafile (you can only shrink the data file by getting rid of the end part of the file so long as it is not occupied by data). In this case, you will probably have to export the data, drop the data, resize the datafile and re-import the data.

If you do decide to delete the datafile, assuming that the tablespace only has this one datafile, first drop the tablespace as in my previous post, then use operating system commands to delete the datafile.