Replace [ID]
with your category id and add it to any layout.xml
for exmplate to app/design/frontend/base/default/layout/local.xml
<CATEGORY_[ID]>
<reference name="root">
<action method="setTemplate"><template>page/2columns-left.phtml</template></action>
</reference>
</CATEGORY_[ID]>
Upd.1
I'l give you few pieces of advice how to debug layout.
In most of cases I prefer to add following construction <remove name="footer" />
to determine is this block workable or not. As result footer should disappear
Add inside <CATEGORY_[ID]>
block from example above (<remove name="footer" />
). This way you can check that you are in correct category
Add inside file local.xml (or some another file which you use) following:
<default>
<remove name="footer" />
</default>
Footer should disappear on every page