How do I correct spelling in a dictionary?

If I use the word agendas, which is a plural plural, the dictionary sees it as correct. I would rather not have to look for these anomalies myself, but allow the spell check pick it up and correct it.
Agendum is the singular form, while agenda is the plural form. Anyone who doubts that should get a legitimate dictionary and check it out.
I need to know how to delete “agendas” from my dictionary.
Thank you!

(I)

Sample: LibreOffice 4.1.2.3 – bundled dictionary dict-en (LibreOffice_4.1.2.3_Win_x86.msi)

See also:

(1) http://www.thefreedictionary.com/dict.asp?Word=agendas

(2) http://en.wikipedia.org/wiki/Agendas#Etymology

(II)

Create a user-defined exception dictionary to exclude “agendas”,

e.g.: language en-US

agendas

In detail: é possivel editar o DB do corretor ortografico ? (Can I edit the spellchecker database ?)

Must be a Windows / other dictionary issue because I just checked the v4.1.2.3 en-GB language pack for DEB/x86_64 and the indicated form is not included. I realise language use changes over time, but this one is rather contentious IMO.

Please indicate your operating system and LO version. A quick check of my dictionaries here indicates there is no such term included e.g.,

$ grep -re agenda /opt/libreoffice4.1/share/extensions/dict-en/
/opt/libreoffice4.1/share/extensions/dict-en/th_en_US_v2.dat:agenda|2
/opt/libreoffice4.1/share/extensions/dict-en/th_en_US_v2.dat:agenda item|1
/opt/libreoffice4.1/share/extensions/dict-en/th_en_US_v2.dat:(noun)|agenda|order of business|list (generic term)|listing (generic term)
/opt/libreoffice4.1/share/extensions/dict-en/th_en_US_v2.dat:(noun)|agenda|schedule|plan (generic term)|program (generic term)|programme (generic term)
/opt/libreoffice4.1/share/extensions/dict-en/th_en_US_v2.dat:(noun)|menu|agenda (generic term)|docket (generic term)|schedule (generic term)
/opt/libreoffice4.1/share/extensions/dict-en/th_en_US_v2.dat:(noun)|fare|agenda (generic term)|docket (generic term)|schedule (generic term)
/opt/libreoffice4.1/share/extensions/dict-en/th_en_US_v2.dat:(noun)|agenda|agendum|list (generic term)|listing (generic term)
/opt/libreoffice4.1/share/extensions/dict-en/th_en_US_v2.dat:(noun)|agenda (generic term)|agendum (generic term)|order of business (generic term)
/opt/libreoffice4.1/share/extensions/dict-en/th_en_US_v2.dat:(noun)|agenda|docket|plan (generic term)|program (generic term)|programme (generic term)
/opt/libreoffice4.1/share/extensions/dict-en/en_GB.dic:agenda/2	1
/opt/libreoffice4.1/share/extensions/dict-en/en_GB.dic:agenda's	15
/opt/libreoffice4.1/share/extensions/dict-en/en_GB.dic:agenda	16
/opt/libreoffice4.1/share/extensions/dict-en/en_ZA.dic:agenda/SM
/opt/libreoffice4.1/share/extensions/dict-en/en_US.dic:agenda/1	1
/opt/libreoffice4.1/share/extensions/dict-en/en_US.dic:agenda's	12
/opt/libreoffice4.1/share/extensions/dict-en/en_US.dic:agenda	13
/opt/libreoffice4.1/share/extensions/dict-en/en_AU.dic:agenda/SM
/opt/libreoffice4.1/share/extensions/dict-en/en_CA.dic:agenda/1	1
/opt/libreoffice4.1/share/extensions/dict-en/en_CA.dic:agenda's	12
/opt/libreoffice4.1/share/extensions/dict-en/en_CA.dic:agenda	13
/opt/libreoffice4.1/share/extensions/dict-en/th_en_US_v2.idx:agenda|324819
/opt/libreoffice4.1/share/extensions/dict-en/th_en_US_v2.idx:agenda item|324995

You may have inadvertently added this spelling form to a user-defined dictionary. In this case instructions for removing the entry can be found in this answer. Also please check that spelling is working correctly for other miss-spelled terms. It may be you have a dictionary with a problem for your particular OS/LO, and if you can demonstrate this (in a manner like that above) then please report a bug under the Extensions component and include as much detail as possible.

As shown the main dictionaries are stored in the share/extensions/ directory and are ASCII text files that can readily be edited.