To stop a flood, simply click on the gear icon when you have the flood selected and click Stop.
The test should stop immediately. However, it's important to note that this does not stop the grid.
The grid is launched and stopped separately of the test and the number of nodes you have running in a grid, as well as how long they're running for, is what determines how much you get charged. Stopping a flood does not mean the grid has been stopped. If a grid is running, you will be charged node hours even if you don't use it to run any tests. This reflects the cost of keeping the machine running on the cloud.
To stop a grid and prevent unwanted charges, click on the Grids button in the toolbar:
Then click on the grid you'd like to stop:
Then click Stop.
You can be sure that you won't get charged if all grids have been stopped.