Untick View>Text Boundaries.
This answers your question but this is a bad idea. All the boundary hints and other marks or shadings you can enable in the View menu are here to help you visualise what’s going on in your document.
They are invaluable tools to optimise and tune your layout. They give you “situation awareness” about what to do (which setting to change) to achieve your formatting/layout goal. Unless, of course, you use Writer like a mechanical typewriter where everything is done manually.
When you’re in the writing phase of your document, it is important for the author to have advanced prediction about remaining space both horizontally and vertically. Formatting marks give you other reminders about your spaces, tabs and paragraph/line breaks. I have noticed that Windows (or Word) is quite lenient about the latter. They seem to be frequently used interchangeably but they other strong consequences in Writer due to the geometrical properties of paragraph styles. (I didn’t use word “permissive” because it would suggest they are equivalent while they are radically different.)
Writer is built around the notion of styles which, except for paragraph styles, are absent from Word. Styles allow for automation of formatting to an extent you can’t imagine before experimenting.
PS: you didn’t mention your LO version; on my 24.8.4.2 under Linux with KDE Plasma desktop, these crop marks are replaced by true rectangles. You may not be on the latest available release.
