The easiest way would be to delete/move your %AppData%\CodeBlocks\default.conf, and let Code::Blocks generate a new one. This will mean that you lose all your settings though.
Alternatively, open default.conf in a plain text editor and search for "<LAST>". You should find a section that looks like
<replace_options>
<LAST>
<astr>
<s>
<![CDATA[goto]]>
</s>
<s>
<![CDATA[operator]]>
</s>
[...]
One of the <s><![CDATA[MySearchTerm]]></s> will have your extremely long string. Delete that entry. (Note: backup your default.conf before any modifications.)