There's your problem. Remedy this and you won't have an issue. 😉

There are a few ways in which you can see the usage of each cube and from that determine which cubes you can safely archive/migrate away from the instance.

The first is a simple profiler trace. You'd have to leave it on for a few days probably and then output it to a table and analyse it. Not pretty but it'll get you there.

You can turn on the Olap Query Log (https://technet.microsoft.com/en-us/library/cc917676.aspx) and let that run for a week or so and then query it to see what is being used.

You can use extended events to capture any queries coming in to your cubes (https://blog.crossjoin.co.uk/2016/04/18/profiler-extended-events-and-analysis-services/) and then analyse the output of that after a week or so.

These methods would involve communication to the users giving them a moratorium of the aforementioned time frames to use the cubes that they need or else they will be archived.

Finally, if you have a list of SSAS users (through a security setting, for example) then you could simply ask them.

Hope this helps 🙂