Assuming that you want to preserve the current layout, do the following:
- Open NinjaTrader Control Center
- Go to File > Workspaces > Save Workspace As.... (assign any name you want)
- File > Workspaces > Open Workspace (select blank and open it)
- Close all charts within blank workspace
- File > Workspaces > Save Workspace
Now you can go to File / Workspaces and you will see the new one you made in the list and the blank one.
If you just want a clean slate quickly, you can open the xml file and wipe it out.
- Open Windows Explorer and navigate to My Documents\NinjaTrader 7\workspaces
- Right-Click "_blank.xml" and Open With Notepad.
- Delete everything in that file, so it's completely empty and save it.