Sheesh, reading some of the posts in this thread makes me glad I work for (and own part of, I will note, so it costs me money if someone takes the piss!) a business that just has a "be reasonable" travel policy. We're not a big company - and I appreciate "be reasonable" is probably impractical past a certain size - but definitely bigger than some of the employers being mentioned in this thread that have super-tight travel policies!
This whole discussion - and our own policy - reminds me of the story in Dave Hitz's book about the changes NetApp made to their travel policy at some stage in the company's life (after they were already quite big). They basically replaced a very long and complicated policy with a really simple statement - I don't have the exact wording on hand but it was something like "be reasonable - we're a frugal company but it doesn't make sense for you to turn up dog tired to a critical meeting. Use good judgement and we'll be OK with it". They (and we!) of course checked how employees were implementing this policy, but at the end of the day there was a fundamental trust that people would do the right thing. Trust but verify!
This type of policy has always worked very well for us - our staff appreciate it, and we've literally never had any issue larger than things that could be a called a reasonable difference of opinion over what "be reasonable" is - definitely no cases you'd clearly call "taking the piss". Most people just ask if they're unsure what is considered reasonable and what isn't. And even in those rare cases where there's been a difference of opinion, it's just been a matter of having a simple conversation with the employee involved to ensure our expectations are aligned going forward - no hard feelings, and we've never had a case where the feedback hasn't been adequately "taken on board". I guess this is all helped by us not being too tight in what our definition of "reasonable" is though.
I'll finish by adding that I think overly restrictive travel policies (strictly defined or not) are really quite stupid - that they probably cost businesses more than they save them in many cases. There's many hidden costs in tight travel policies - tired and/or stressed employees don't perform as well (which obviously costs money), unhappy staff will leave the business sooner (which costs money - often
lots of money), there's a lot of intangible (but at least partially monetary) benefit in having employees know that the business trusts them, etc.
EDIT: I should have added that the "be reasonable" policy clearly only works when there is some monitoring, and senior management crack down on abuse if it happens. OP, it sounds like your current policy is something like ours, but is now breaking down through non-enforcement. So clearly my post isn't aimed at answering your question - it's just general chit-chat!
Referring to the op, you be the judge -
To put the above in context, I thought I'd perhaps post my take on this - noting this opinion is "in isolation" and ignoring all the others issues you posted about this bloke!
1 hour meeting at Crown Melbourne - Valet Parking $50
Very unreasonable unless he was late by genuine accident or for reasons outside of his control, or genuinely wasn't aware that much cheaper and almost-as-convenient parking is readily available.
Flight Mel - Syd (daytime during office hours) - short term car park MEL (the one that you usually just drop off or pick up from) $115
Reasonable for a more senior person / employee who is highly regarded assuming this is cheaper than taking the trip by taxi, which is quite likely in MEL.
Flight Mel - Syd (no checked baggage) $410 (this was about $135 more than best fare)
Hard to say without knowing why he didn't take BFOD - from very unreasonable is there was
no good reason / he simply didn't bother to book until last minute, through to reasonable if there's a decent reason (of course!

). Something like a preference for QF over DJ, assuming there's some logic behind it (e.g. has a QP membership but no DJ lounge membership, or is QF SG+ but no status on DJ), would put this is the pretty reasonable category for me - this level of fare difference is about the max I'd tolerate and I'd like to be asked first though.
Car Hire in Sydney (56kms travelled - 1 person only) - SV6 Holden $138
Based on distance travelled, a bit unreasonable but depends on mitigating circumstances (e.g. ability to accurately predict distance and relative costs before the trip). Based on vehicle choice - for a more senior person / employee who is highly regarded I wouldn't call this unreasonable, but I'd like to be asked about it first.
Meal on the way home from airport at about 5.30pm at Cafe approx. 15 mins from home $51.80
Completely unreasonable - hard to view this any other way.