The options for managing tablespaces have got a lot better over the versions starting with 8i. This is especially true if you are using the appropriate types of file for a temporary tablespace (i.e. locally managed tempfiles).
So, it could be as simple as this command, which will shrink your tablespace to 128 meg...
alter tablespace <your_temp_ts> shrink space keep 128M;
The Oracle online documentation is pretty good. Find out more.
edit
It would appear the OP has an earlier version of the database. With earlier versions we have to resize individual datafiles. So, first of all, find the file names. One or other of these queries should do it...
select file_name from dba_data_files where tablespace_name = '<your_temp_ts>'
/
select file_name from dba_temp_files where tablespace_name = '<your_temp_ts>'
/
Then use that path in this command:
alter database datafile '/full/file/path/temp01.dbf' resize 128m
/