My first thought was to disable the spell checker in Evolution.

A Google search using "ubuntu evolution dictionary" has a LOT of hits.
You might want to look at some of them.

It seems, though, Evolution "should" be using the standard system
dictionaries. It's possible some of them aren't installed on your
system.

I just brought up Synaptic on my 8.04 system, clicked the "Search"
item, and entered "dictionary" as the search term. There are 13
selected and installed on my system.

What you should do is start Synaptic and do the same "dictionary"
search and examine the list of candidates for installation. Anything
ending with "-en" or "-eng" should be selected along with any other
dictionaries that might seem useful (e.g., computer acronyms, etc.).

The packages installed on my 8.04 system appearing in the search are:

aspell-en
dictionaries-common
gnome-utils
iamerican
ibritish
ispell
kbabel
libgdbm3
myspell-en-gb
myspell-en-us
myspell-en-za
wamerican
wbritish

These could also be installed per (for example):

sudo apt-get aspell-en
etc.