If the referendum is invalid, even more reason to let it happen if you can't make it go away. Ignore it, make sure those friendly to you promote the idea of participating especially if you are against it. If "leave" doesn't have a clear victory, you can point to it for why you are against them leaving. If they do, your position is at least not worse than it is now, and probably better since you haven't burned most of the goodwill people have towards the government.
(It's my understanding that the laws passed by the local catalonian parliament that said things would happen as a consequence of the referendum result were thoroughly voided before the referendum due to being against both spanish and catalonian rules. At that point, one should be able to treat the referendum as just a poll to inform future politics)
Maybe you are right, but the scenario is this:
1 a group of people organizes a referendum or something similar
2 someone else complains at the Constitutional Court or whatever is the institution that has(by law) decide if the act is constitutional or not
3 if the thing was declared illegal the institution that has (by law) stop the illegal thing must act
My country has a similar constitution, a region can't separate unless the constitution is changed. Probably this is not fair for some regions but it is how it is, you can't decide to ignore constitution, not sure how this is in your country. I was hoping that in future EU would unite us and we get rid of the nationalistic views and finally borders.
(It's my understanding that the laws passed by the local catalonian parliament that said things would happen as a consequence of the referendum result were thoroughly voided before the referendum due to being against both spanish and catalonian rules. At that point, one should be able to treat the referendum as just a poll to inform future politics)