There is workarounds for simulate single shot, or force AI servo (in some cases).
To simulate single shot, just focus, and when lock is achieved, switch to MF to keep it there. Not the best solution but if works.
To force AI servo, there is 2 ways, a good and a bad one. In fact it's the same solution, but is good or bad depending the lens.
When the camera is AF locked, and you slightly defocus the image, tha camera turns to AI servo. If you have a FTM lens, you can do this changing the focus (go to a far distance works better to me in my tests with the 28-135 IS) This is the good way. The bad way is do the same in a non-FTM lens, as the 18-55, sligtly touching the focus ring but without actually moving the motor (because it's locked by the AF switch). The touch tends to defocus a little the lens as in the FTM case, and the camera switch most of the time to AI servo. This is the bad way because is not as reliable as the FTM alternative, and probably isn't the best for the focus mechanism in the long term.