are admins able to block them from being used on 04scape?
Blocking 3rd party clients is a lot like trying to block hackers from connecting to a website. You can write software to detect illegitimate requests, but ultimately if the other party’s software is speaking the correct protocol, there’s really not anything you can do to tell it’s not a real client.
Other online games try to do this with anticheat engines that hash the client software to make sure it’s a legitimate build, but you can’t really do that for a browser-based game, and that’s also very invasive to people’s computers so they often take issue with games that require it.
No blocked but detected and you’ll be banned
In theory, a properly made client would be undetectable so long as it isn’t a bot client or something that’s sending clear signals that it’s not being controlled in the usual manner.
That said, if anyone reading is considering it, consider further: do you trust the developer of a 3rd party client (or yourself) to do it correctly enough to not be detected?
While I do have a pretty good idea how Lost City detects 3rd party clients, I can’t share that so I’ll share something else.
I know for a fact some of the largest social media apps in the world (think YouTube, TikTok/etc) know who the bot accounts are, but they deliberately don’t ban them instantly because they don’t want the bot developers to know how they’re getting detected.
Feel free to extrapolate that to the people who get banned only after they hit level 80 on Lost City 
Most people don’t know this but you can just create js code right on the browser itself. This is completely undetectable. But if you create bots and such, well yes the result of using them, is “detectable”. There is always some sort of manual review in the end. You can use things like Tampermonkey scripts to make your life easier. This can be done on any website and such, it is just JavaScript on the browser afterall.
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Are there any that exist currently?
Yes, Trent (from Darkan) made one and was banned. That’s where I got context for Lost City’s anti-bot detection
(His client was similar to how Jordan described, injecting actions to the existing JS client)