Is there a way I can block certain parts of my site without upsetting Google?
If youre gonna block certain parts to google, you might as well do the same to the public, otherwise you're asking for trouble from google. In your situation, I don't think any kind of blocking is necessary.
island" is coming out as the most used word on my site, because of the dropdown list. How do I stop this from happening?
Make island a category and list it as such. So if your items are this:
Jacks Island
Manitoulin Island
Darkmans Island
Blueboys Island
...
Then change it to this:
Islands
Jacks
Manitoulin
Darkmans
Blueboys
But style the category differently so that people won't think its a clickable menu item. and if your list is inside a listbox on the site, put the category name (islands) outside of it so people can always see the category as they scroll through the list.