>This isn't a trunked system
Then make it one.
>encryption is illegal
It's not illegal and is a core part of modern radio protocols.
most equipment doesn't use trunked protocols like DMR, because it's vastly more expensive, and more difficult to use that honest-to-goodness analogue radio.
Plus, the hypothetical we're discussing here is that some non-ham asshole left a spurious transmitter running. Do you think that a trunked magically protects people from interference? No.
Finally, yes, encryption is illegal to use on ham radio.
> It's not illegal and is a core part of modern radio protocols.
LOL, yes it's for most practical purposes on hobby ham bands.
If you wanna be pedantic about it, the FCC rules for ham operations doesn't mention encryption, it mentions encoding instead [0]. So yes, if you encrypt or encode without obscuring the meaning (by publishing your encoding spec or encryption keys) then yes you can "encrypt". digital mode radio works that way.
[0] https://www.ecfr.gov/current/title-47/chapter-I/subchapter-D...