I also couldn't identify the TABTIP.exe application while running, and trying to winkill the pid does not work - UNLESS you run your AHK process as Administrator - but killing the USER tabtip.exe process makes TABTIP rather unstable.
So... and this is a REALLY sucky way to do this, but it works... :-/
Code:
XButton1::
CoordMode, Mouse, Screen ; Set us to screen co-ord mode
MouseGetPos, nX, nY ; Get the current mouse co-ords
SysGet, Mon1, Monitor ; Get the screen resolution
MouseClick, Left, Mon1Right - 47, 8 ; Click the X button for the TIP
MouseMove, nX, nY ; Restore the mouse to it's original co-ords
return
I've simply left-clicked the X button of the TIP (47 means that you shouldn't click the X button of another window by pressing XButton1 more than 1 time by mistake), and 8 is approximately in the middle of the X button height.
Like I said, it's not elegant, but it works...
Naturally, if your TABTIP is docked at the bottom of your screen, you need to modify the X co-ord ( 8 ) to some valid co-ordinate.
If your tabtip is a floating window, well... work it out...