Meh. I've only fairly recently gotten into GMRS given how the one license covers everyone in the household, and frankly there seems to be a lot of GMRS operators that would be much happier as hams or much happier with cellular PTT radios. I've heard people complain because other people were operating, like they had some sort of exclusive claim to a frequency, and this isn't the first complaint about repeaters that I've heard. Other complaints have revolved around insufficient remaining channels available for repeaters in a given area, basically channel congestion.
The amateur service is by no means perfect, but there's enough spectrum even on just 2m and 70cm to where repeater congestion isn't causing problems.
How does that work? Non-standard tone? Surely people just try them one by one?
The most common way is split tones. Transmit and receive different tones. tx DCS and rx CTCSS or vice-versa. The users would need radios with that capability or just use the TX tone and receive CSQ but that confuses or upsets people that think they need a RX tone. The TX tone is "secret". In may cases that's good enought to keep out the riff-raff until they find out that all you have to do is listen to the input and find the tone the instant somebody transmits.
You could use a non-standard tone if your repeater controller or tone board has the capability but the user radios would also need that capability. That works unless you picked a tone that was within the tolerance of a standard tone. You might use 100.1 Hz and wonder why people can access the repeater with 100.0 Hz.
You can manually program a Baofeng for any CTCSS frequency between 60.0 and 259.9 Hz in 1 Hz steps and they do split tones.
You need a brain and patience to do that.
I'm not sure how split tone is supposed to help when a receiver doesn't have to use tone-squelch at all in order to listen. It's convenient so to not open on receiving any other transmissions from other stations on the same frequency but it's not going to somehow prevent a recipient from hearing the repeater output. And I suspect that a lot of those using repeaters don't even bother with configuring tone-squelch even if the repeater offers it.
As for the TX tone for opening up repeaters generally regardless of service, my FT-5DR can receive and tell me what the tones being transmitted are even on frequencies that are out of its TX range. If another station opens up the repeater and I can hear that other station directly on the repeater input frequency then I'll be able to figure out the tone that was used.
The only way I see a club managing to reduce problems on their repeaters would be to cycle the tone on an almost constant basis, basically requiring their members to change to the newly-configured tone on a schedule, and doing it frequently enough that it annoys would-be jammers sufficiently to where they just stop bothering.
This is one disadvantage of not having something like the ARRL, there's no advocacy body that will at least try to organize its members to sniff-out the offender to forward on to the FCC. That still doesn't guarantee that something would come of it but I expect that getting the FCC's attention to bad-actors is more difficult on a service that's bordering on being the wild west and anyone with thirty-five bucks can get a callsign.