Oh dear - sorry about your poor little gerbil. Unfortunately cats and small rodents generally don't mix and other than keeping them in separate rooms I don't know how you will be able to stop Coleo from going after them.
Other than telling him off at the time I don't think you can punish him as it is just a natural thing for him. You could not give him any treats for a couple of days but I think that might only make you feel that you had done something to punish him - sorry to say, he won't notice at all.

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