Edit

Checking the comments and other answers it looks like some are still looking for toggling touch using the tablet's buttons. If you just want that you need 3 things (shown for Ubuntu/Unity):

1. A simple command (that can be run in the terminal or in a script file)

randompast's solution with a 1-line command is nice and can be turned into a more convenient 1-line toggle command:

xsetwacom --list | grep --line-buffered "TOUCH" | awk '{system("echo "$8";xsetwacom --get "$8" TOUCH;")}' | tr "

" " " | awk '$2 == "off" {system("xsetwacom --set "$1" TOUCH on")} $2 == "on" {system("xsetwacom --set "$1" TOUCH off")}'

You don't need that if you already have your own script or command line of course, but it's a convenient way of toggling the touch of connected tablets without knowing the tablets' name or id.

2. Set up a keyboard shortcut

If you want to run this without the terminal, you need to save the script in a file, make it executable and create a keyboard shortcut for it. That's because you can't assign a command directly to the tablet's buttons (but you can assign a keystroke ...).

In Unity open System Settings > Keyboard > Shortcuts and create a new shortcut. Avoid the Super key since it may not always work in step 3.

As command, put the name of your script file (full path, between single quotes, if you're not sure then drag-drop the file in a terminal and use the created command).

3. Assign the shortcut's Keystroke to your button

Open System Settings > Wacom Tablet > Map Buttons ... and assign the keystroke you chose to your button.

To know which button is which you can first assign them letters with no modifier, select a text field and press the buttons to see which letter they correspond to.

Here beware: not all the buttons will be recognised (3 out of 4 for my Bamboo) and not all keystroke will work (I can't get any with the Super key to work). You'll have to try different combinations.

Original Answer (to learn the commands)

I set up my Pen & Touch over a year ago so I may be wrong here. What I usually do is run in a terminal:

xsetwacom --set "Wacom BambooPT 2FG 4x5 Finger touch" touch off

This won't work for you as you have another tablet so try first

xsetwacom

which should give you the parameters to use. First you need to get the name of your device (let's call it DEVICE_NAME), so try:

xsetwacom --list devices

This gives me for my Pen & Touch:

Wacom BambooPT 2FG 4x5 Pen eraser id: 11 type: ERASER

Wacom BambooPT 2FG 4x5 Pen stylus id: 12 type: STYLUS

Wacom BambooPT 2FG 4x5 Finger pad id: 13 type: PAD

Wacom BambooPT 2FG 4x5 Finger touch id: 14 type: TOUCH

Obviously the one which is of type TOUCH is the one to disable here. Here DEVICE_NAME would be Wacom BambooPT 2FG 4x5 Finger touch and DEVICE_ID would be 14.Then you need to know which parameter to disable, so try:

xsetwacom --list parameters

There's a plethora of parameters to enable/disable, and for each parameter name (let's call this PARAM_NAME) the type of value expected is explained. In this case we want to turn the Touch parameter Off, which you can do with the --set option. The syntax is:

xsetwacom --set "DEVICE_NAME"  PARAM_NAME  PARAM_VALUE

or

xsetwacom --set DEVICE_ID  PARAM_NAME  PARAM_VALUE

If you're not sure what to put in place of PARAM_VALUE, try the --get option to see the current value:

xsetwacom --get "DEVICE_NAME"  PARAM_NAME