I'd vote for a one line entry subtracting out all of the improper time with a short note in the memo field along the lines of "Correction - removed inadvertently entered flight time." If anyone ever notices and asks, just answer honestly about the whole situation and explain that this motivated you to become more familiar with the FAR's so as not to make a similar mistake.

We all get things wrong starting out - fix the problem and be honest if anyone questions it and I seriously doubt anyone will make an issue out of it (aside from the tools United used to have doing their interviewing).