I don’t think we have a setting to disable this one… you can hide all composer popup messages entirely with CSS (added to admin → customize → themes)… we already do this on mobile devices, so you wouldn’t be hiding anything critical by doing this:
.composer-popup-container {
display: none !important;
}
The similarity warning can be disabled by changing the max similar results site setting to 0. Though, worth mentioning that we use it on our own internal community and it does prove useful from time to time.