I was looking for an answer that works in Microsoft Word for Mac. I tried all the answers and none of them worked. Here is the procedure that worked for me:
1- Copy a sample piece of text to the clipboard.
2- View > Macros > Choose Record Macro...
It will open a new window.
3- Click the "Keyboard" button in the new window.
4- Press Ctrl + h (or whatever combination of buttons that you prefer.)
5- Click the "OK" button.
6- Use Ctrl + v to paste the copied text to somewhere in your document.
7- Using your mouse, choose paste options, and click on "Keep Text Only".
8- View > Macros > Stop Recording
From now on, whenever you want to paste anything with the "Keep Text Only," instead of Ctrl + v, you can just press Ctrl + h (or whatever combination of buttons that you prefer.)