I would advise you to go and change your control file with new maxdatafiles limit than try dropping unused datafiles. The pain you will be taking for dropping datafiles(unused) will be much more than recreating controlfile, even from the risk perspective and flexibility.

What if tomorrow you run outta space again ? that time you will not be left with unused datafiles to drop and accomadate new datafiles. So, Best bet is change maxdatafiles limit by recreating control file.

If you want steps search for 'rename database', you have bunch of postings on how to recreate controlfile.

Remember! you don't want to rename database but redefine maxdatafiles limit while following the steps in postings...