First a misconception:
any selected text is immediately sent to the clipboard
Actually text is never "sent" anywhere until it is requested by a receiving application. When you select text, the application only claims the selection, which means basically that it raises a flag to say that from now on it owns it.
Now on to your question:
In X11 there can be multiple selections. 2 of them have well-known names and are standardized. They are called PRIMARY and CLIPBOARD. Their respective conventional behaviors are as follows:
- Applications claim PRIMARY when text is selected
- Applications request PRIMARY from the owning application and paste its contents on middle click.
- Applications claim CLIPBOARD when an explicit command is given, typically Ctrl-c.
- Applications request CLIPBOARD from the owning application and paste its contents when an explicit command is given, typically Ctrl-v.
- There might be additional rules I'm unsure about, like if no application owns CLIPBOARD but some application owns PRIMARY, paste primary instead upon Ctrl-v.
It seems like CLIPBOARD already does what you need. You can ignore PRIMARY if you want (but note that some older applications like
xterm may only support PRIMARY). Personally I do the opposite: I ignore CLIPBOARD and use only PRIMARY. I guess that's just the way I learned to use X11, I wasn't even aware that there was CLIPBOARD at first. But in order to mitigate the problem you describe, I often wish there was a pushable & poppable stack of PRIMARY selections, so I could "pop" to the previous selection after clobbering it with a different one.
In response to your explicit question about whether the PRIMARY behaviour can be disabled, I think that would be quite difficult. The most straightforward way would be to individually disable it in each application (or toolkits which the applications use) which is surely not feasible. I suppose a kind of "X11 firewall" which blocks requests to claim PRIMARY could be constructed, but I don't think that would really buy you anything more than you can already get by ignoring PRIMARY and using CLIPBOARD only.
More information: What's the difference between Primary Selection and Clipboard Buffer?