It happens to us all. Something isn't quite right and we need to abort a stimulus run.
If you are using Spike2 on the MASTER computer, then click the STOP button (the one in the toolbar near the graphs, not the one that is in the upper left). Then click the QuitRestart button to reset the Spike2 script and to setup for the next acquisition.
On the STIMULUS computer, bring the command window to the front and hold down the SPACE BAR; if the space bar is being held at the end of the presentation of a given stimulus, the sequential presentation will stop and the computer will return to its state of listening to remote commands. Often, you'll generate so many spaces that the window will scroll over to the right. After this happens, press enter a couple of times to return to the left side of the command line. You should see the computer saying "Waiting for remote commands..." when it is ready to continue.
If you are acquiring on the 2-PHOTON computer, click Abort T-series on PrairieView (or whatever software you are using).
If you are acquiring with an intrinsic imaging system, go the IMAGING computer and click the STOP SIGN (it's an image of a stop sign in the toolbar on the top of the window) to stop acquiring (if it is a LabView system) or click "Stop" or "Abort" if you are using a system from Optical Imaging Inc.