Another possibility is that you just aren't clicking in the right place. Tab
markers can be very tricky and annoying for new users, or people who haven't
perfected mouse skills. Microsoft didn't help matters either by making the
tab markers so darn small, and for some reason if you click in the upper
section of the ruler, you cannot move/clear/set tab markers. The mouse cursor
must be in the lower portion of the ruler (still on the white part, but lower
than the little ticks between numbers) for it to work. I have taught quite a
few beginners how to use tabs, and invariably someone is "unable" to set tab
stops because they are clicking just too high. As for moving and clearing, if
this is the problem, just keep trying, you'll get it eventually. You have to
click on the vertical part of the "L"; the horizontal part will just set a
new tab. It is only a tiny little area to click in, so you need a steady hand
and patience. When you drag it down, it will turn grey (but stay on the ruler
as long as the mouse button is held) and the vertical dotted line will
dissappear. When you release the button the tab marker will be gone.
Good luck!