These blocks only appear after you have browsed other products within the store.
To remove this feature go to:
app/design/frontend/default/your-theme/layout/reports.xml
And,
app/design/frontend/default/base/layout/reports.xml
Remove the following code. Note that this code might not be present in your theme and only in the base.
<block type="reports/product_viewed" before="right.permanent.callout" name="right.reports.product.viewed" template="reports/product_viewed.phtml" />
