In the MusicBee installation folder there is a file called "Configuration.xml" - open this in any text-editor and search for the word "occasion", and you'll see a section list like this:
<Occasions>
<Occasion Name="Background"/>
<Occasion Name="Birthday"/>
<Occasion Name="Breakup"/>
<Occasion Name="Chillout"/>
...
</Occasions>
Just delete all the "<Occasion Name=..." lines you don't want (you can remove them all, if desired). If you remove them all, be sure to leave the section-elements themselves:
Save the changes, restart MusicBee, and they'll be gone from the dropdown. Be sure to back up the original Configuration.xml file first, "just in case of fire".
