With that said, it is actually standard practice to use katakana for on-yomi, not just here on renshuu but virtually everywhere (at least, it is the convention I mostly encountered during my studies, there might be individual exceptions of course). One benefit of using hiragana for kun- and katakana for on-yomi makes it automatically clear what kind of yomi you are looking at (of course furigana are mostly in hiragana, regardless of the used yomi).
Finally, it certainly makes sense to learn katakana as there are an awful lot of words in the Japanese language that are written in katakana only and not being able to read katakana can and will prevent you from understanding texts that you will encounter in everyday reading situations (newspapers, manga, television with subtitles, ...). I believe standard recommendation is to learn hiragana and katakana first and then continue with learning Kanji, but there of course is nothing to prevent anyone from mixing this up and strew in some Kanji's in between.
So maybe rather see this onyomi in katakana inconvenience as a chance to practice your katakana reading skills to get those memorized even faster? :)
SirEdgar is right - there is no way to disable that.
I've been studying Japanese for 15 years, and I still don't like katakana :(. It is 100% required, though, in order to gain fluency, and as SirEdgar noted, this is the standard way of writing onyomi readings in dictionaries, other sites, etc.
If you look up katakana in Learn > Study Center, we do have some katakana decks you can use for studying. They appear under "Alphabets".
