You are certain to get a lot of keywords from automatic generators that are not grammatically correct. Also there will be keywords that are not relevant to your brand's product that you are targeting.
Remove keywords that don't make sense in a sentence. For example, 'network security how to' and 'network security basics youtube'.
Remove keywords that are branded. For example, 'apple network security' or 'microsoft network security'.
Remove non-evergreen keywords. For example, 'network security best practices 2012'.