Pruning the registry is about the only way to banish context menu entries.
There should be an option to write or remove them IMO, and you are not the first to note this annoyance.
RUN "regedit" and do an EDIT|FIND for "convert video file." Delete those instances.
Back up the registry or create a system image first.
Registry edits are effective immediately and mistakes can hose your operating system.
Another approach.. http://www.softpedia.com/get/System/OS-Enhancements/ContextEdit.shtml