When your Dropbox has reached its maximum capacity, you can download the event videos onto your computer and/or remove them from your Dropbox in order to free up space.
You can also get more space on Dropbox with Dropbox Pro. This will allow you to store more videos. Go to Dropbox website to learn more about it.
If you no longer wish to use the Dropbox storage service, you can disable it from the Home + Security application settings: Cloud Storage> Dropbox.