Words you added to your personal dictionary can be removed from your dictionary at Advanced - Configuration - Spell Checker - Dictionary.
The other, pre-installed dictionaries are compiled (binary) and therefore you can't easily change the content of these dictionaries. There is a file named README in the GNU\aspell subdirectory which you should read and introduces you into the world of Aspell.
Best regards from an UC/UE/UES for Windows user from Austria