How do I get rid of that annoying text box in the plot window?

Turn it off by disabling the pointerinfo plugin (how do I do this?).   You can move the box by dragging it with the left mouse button, unless the pointerinfo plugin has "Info window follows the mouse" checked, in which case the box moves with the mouse. 

The text box reports information about what is under the mouse location.  You can configure some of the items which it displays from the pointerinfo menu.  You can also ask that the box info be automatically copied to the clipboard each time it changes.  In that case, by using Alt-Tab to switch to an editor or spreadsheet, you can paste the data into a document without moving the mouse (and thus changing the data in the box).

For users familiar with R:  you can modify the format of the box and/or the clipboard data by changing the functions in the formatter and clipboard.formatter items of the file:
radR/plugins/pointerinfo/pointerinfo.conf.R