To uninstall the program, drag DfontSplitter from Applications (or wherever you placed it) to the Trash. Then empty the Trash.

You may also wish to remove the Preference file that DfontSplitter uses to store information such as the last folders you used and window placement information. This file is located at:

~/Library/Preferences/uk.org.upfold.dFontSplitter.plist

Find this file, drag it to the Trash and empty the Trash.