I don't think the question is stupid. Dismissing it out of hand seems more so.
1. Asking questions such as this, where we have limited information, often spawns interesting approaches to solving them
2. Any method for detecting 'unwanted visitors' may also be effective in detecting unintelligent (but still unwanted) visitors like significant lumps of fast moving rock which if unencumbered may cause an extinction event
3. It is an opportunity to involve people across national, political, tribal and ethnic divides in pursuit of something important to all of us.
(I'm sure there are many more advantages to at least contemplating what our civilisation could do in this 'hypothetical' situation but this lot should do for demonstrating that the question is at least worth asking...)