There should be an option in Systemsettings > Mouse. If enabled, middle clicking on text entry boxes should not paste the content from the clipboard into the text entry box. Note that the middle mouse button should continue working for other purposes, for example to close a tab. This entails all KDE applications, but not necessarily other applications run on the KDE desktop.
Which text entry box? If this is about the QLineEdit or QTextEdit, then this needs to be added in Qt, but Qt tries to avoid user configurable settings.
(In reply to Christoph Feck from comment #1) > Which text entry box? All of them, in all KDE applications. I can't really add a comprehensive list here. Just some examples. * Konsole terminal commandline * Dolphin adressbar, renaming text entry, all the text entry boxes in configuration * Any text entry box in systemsettings5 * Kcolorchoser color entry box * Save as dialog filename * Kalarm new alarm etc.
Konsole has its own text field, and there is an option to switch MMB between 'Paste Selection' and 'Paste Clipboard'. A third option to disable the MMB could be added. For all other applications you listed, these simply use QLineEdit. Please report this wish directly to Qt developers via https://bugreports.qt.io/ Reassigning to Konsole developers.
Is there any standard for not allowing MMB to not paste? It seems like a lot of non-KDE applications allow it (xterm/chrome/firefox). I would suggest searching for "linux disable middle mouse button paste" and trying those.