I deleted some files but they are recoverable can they be made irrecoverable

http://download.cnet.com/File-Shredder/3000-2144_4-1066...
Try that!

The best way to make the files un-recoverable is to remove the drive, place it on the sidewalk, and take a sledge hammer to it. Guaranteed to work. Otherwise your bits & bytes are recoverable, as you have found out.

Actually the more I read about Shredders the more I agree with ksiemb, seems that one not too clever with jpegs etc. according to some reviews. UBCD's format & write 'ones' throughout I imagine would work?

you can try ccleaner's (from piriform, same company that made recuva) free space wipe option. to try this,
open ccleaner (latest version) > click 'tools' > 'drive wiper' > from the 'wipe' section, select 'free space only', additionally you can specify the number of overwrites from the 'security' section > select the drive's checkbox from drive list > click 'wipe'.
caution - do not select 'entire drive', it will format the whole drive including existing data.
you can also try darik's boot and nuke (dban) to wipe entire hdd. make sure you back up your important data beforehand.