Got To Preferences > Plugins > Device Interface Plugins then highlight the PRS-300/505 plugin and click the Customize plugin button. This pops up a window that lets you choose the formats and in what order of preference. You can simply move the LRF up past epub.
However if there is an epub I think it will still select that. I think if you uncheck all the other formats Calibre will then convert everything to LRF. Anyway, this is where you get to play with the formats you can accept for your reader.
Hope this helps.
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