A running program can be interrupted in various ways:

Interrupting

Pressing the "Interrupt"-key ("Untbr" on german keyboards) in a window will interrupt that window's process and open a debugger on it. For those used to squeak or visualworks smalltalk systems, the "Alt-." (alt-period) key-combination also shows this behavior.

Leave the debugger, either by proceeding, aborting or terminating.

More information on the debugger is found in the tutorial and in the debugger's documentation.

Aborting

Pressing the "CTRL-Interrupt" ("CTRL-Untbr" on german keyboards) will abort the running process. This is the same as an interrupt followed by aborting in the debugger. The process receives the Abort-signal and is thrown back into its main event loop (or doIt command-loop, if it is a workspace).

Interrupting on the Console

A runaway non-GUI process can be interrupted by pressing "CTRL-C" on the console (xterm) window. However, ST/X must have been started with a console window beforehand ("stx.com" vs. "stx.exe").

Using the Process Monitor

If you have a access to a process monitor window (or you can open one in the launcher), you can also interrupt, stop, abort or terminate any other process from there.

Breakpointing

If you have a access to another browser window (or you can open one in the launcher), you can also place a breakpoint on a method which is known to be called by your runaway program. I.e. you don't need to stop a program for a breakpoint to become active. However, this only works if ou have any idea on what the runaway process is going to do.