Yes, from the command line.
Ctrl-C brings up a list of processes, you can choose the number of the process and kill it. 
If I do it two often it sometimes gets confused but it has got me out of trouble before. 

On 25 May 2015, at 2:45 pm, Chris Double [self-interest] <> wrote:

Is it possible to interrupt a UI action? I accidentally clicked
"implenters" on a slot named 'parent' and the UI froze for a long time
while it was computing. Can this be canceled in any way?

I also then clicked the 'expand' icon which took even longer and
eventually crashed, probably from an OOM.