View>Grids and Guides> Hide Guides
Unfortunately the Purple one is linked to the blue ones (ruler guides) as well as other ones, you can't turn just one off, as far as I know.
These are just Guides and don't output unless you tick the "Print Visible Guides and Baseline Grids" in the File>Print dialog box.
I'm assuming you are talking about the column guides (not margin guides, eh Bob).
Adobe insists on having both margin and column guides in the document, it's been like that since PageMaker. I've never been able to find a practical use for the movable column guides that didn't seem a bit contrived, but if you add a couple of columns to your layout, you will likely see the logic. The magenta margins are page margins and as such constrain page elements, the purple column guides contrain the text.
As Bob mentions you can change the colour of the column margins to be magenta the same as the page margins, then you wouldn't notice them, but that would likely lead to more confusion. The best solution is to accept them for what they are, and ignore them if you don't find them useful.
And by the way, it's not just the columns and margins that are linked in the display: all document guides, including bleed and slug, are similarly linked.
Purple, magenta? What’s a bit a red between friends? J
I had exactly the same problem and even found this thread in my Google search.
I solved it myself by clicking on nearly every function.
To solve the purple lines from appearing all over your document, quite simply:-
1. Look top left hand corner where you can see the emblem of Indesign - id
2. On the right of the emblem you will see 5 icons, the first one is Br which is Bridge - then 4 icons with drop downs
3. Got to the second icon and click the drop down, untick Guides and then they will disappear.
Simple! Job done!
You can get the page and column guides out of sight by changing the screen mode. View > Screen Mode > Preview (instead of Normal)