On the toolbar there is a button labelled with a red cross. On several of the plugins there is a button labelled cancel.
Most operations in Mandelbrot Explorer can be cancelled part-way through. If you are running such an operation, these Cancel buttons will be enabled; click on one of them to cancel. The operation will cancel after a few seconds.
If Mandelbrot Explorer has calculated a new fractal region to display, but was part-way through colouring it in when the operation cancelled, Mandelbrot Explorer needs to display the calculated region but has not worked out what colours to use. If this happens, a temporary colour-scheme is applied to allow the region to be displayed. Mandelbrot Explorer shows that a repaint is needed (to paint the region with the correct colours), so you can click "Start" at any point to colour the region in properly. This temporary colour-scheme is pictured below: