ORACLE 8.1.6

I have this weird scenario,
I have drop a tablespace that is used by an RBS. I intend to recreate a bigger TABLESPACE for a bigger RBS.

After I drop the tablespace, I issue
the command in o.s.:
rm RBSbig01.dbf

the command was successful because it didn't feedback any error.
but to my surprise, the 500M size of that file didn't reflect the AVAILABLE size as i issue a
df -k

it was still the same before I remove the datafile.

Any experience of such kind guys, pls share, as again I am very much short of resource the only remaining file system that I can use is the one where ARCHiving resides?

Thanks in advance.