Hi t.kyhn,
To add to what Kappy said,
The dictionary file is called LocalDictionary and is located in your user Library folder and is a regular text file.
In Finder, go to the go menu in the menu bar: Go > Go to Folder
Type ~/Library/Spelling in the Go to the folder box and click Go.
In the Spelling folder, right-click on the file LocalDictionary and come down to Open With > TextEdit
The LocalDictionary file will open in in TextEdit.
Each word will appear on a separate line. You can then delete words just like you would when editing other text files. Make sure they are kept on a separate line
When you are finished, go to the file menu and come down to Save or press command s to save the LocalDictionary file.
I hope this helps.