The simple answer is you can't disable it through the UI or XML. The drilldown is bound to each marker's 'click' mouse event in the code. The app author would need to expose a setting in the format menu and toggle the drilldown based on the value.
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I think custom viz apps are inherating certain properties from more general vizualisation elements such as . I'm not an expert but anyway, the following worked for me to disable the "timeline" default drilldown :
Under the element of your source XML, just add a dummy drilldown which sets a token, for instance:
<viz type=...> ... <drilldown> <set token="sourcetype">1</set> </drilldown> </viz>