I don't think that compulsory membership would prevent cache-trashing. Our only cache was muggled in March and something nasty left in it. The culprit even signed the log, and it was clear that the previous finder was observed (from a distance) replacing the cache. I think the solution is to take care you're not seen retrieving and replacing caches, and to hide them well. We would rather walk on and return later if there is any danger of being overlooked, as we know how upsetting it was to have our cache trashed.
In many cases, you'd need to be pretty determined to find a (well-hidden) cache from the website without a GPS. Of course, a 1/1* plus the hint/spoiler pic might be fairly easy, but you'd need at least an OS map for the harder rural ones, and even then it might take a while. If someone is that determined to make mischief, they'd just stump up the membership fee.
Most people who own a hand-held GPS will probably know about Geocaching anyway - we found out about it because our etrex had an icon with the website address. We bought ours originally for hill-walking, and I suspect that a lot of others do the same. IMO, if you enjoy the outdoors and are willing to spend a fair amount of money on a GPS, you're not likely to be the sort of yob who goes around vandalising things for fun.