had acccdentally sent a wrong amount to the wrong address, and had to cancel out by betting proportionately on the other two sides. This caused my ROI to drop. Well, I didn't lose much, so I am fine.
Well, same happened to me.

The competition hasn't started yet, so there is no problem there, lol.
If the bet was placed before say 1 hour or something before the match I think you can contact
support@directbet.eu and request them to cancel the bet. But a better idea is to bet proportionately on all other things and request to cancel all 3. That way even if something wrong happens, you won't lose the entire amount.