I bound Alt+Space to an action, for example:
!Space::Send, Foo
Now how to trigger the original Alt+Space's action (as shown in the image below) through code?
To distinguish between user keystrokes and "artificial" ones you have to make use of keyboard hooks.
These hooks are typically setup automatically. (Although there's the possibility of manual changes with #InstallKeybdHook)
But even if available they are not always in use. To force them you can use #UseHook or the modifier symbol "$" in front of hotkey definitions.
An example related to your question:
$!Space:: MsgBox, Action configured with hook.
F2:: Send, !{Space} ; This can still trigger the old behavior.
A different approach would be temporarily to turn off your hotkey through the Hotkey command.