Echoing /u/somewhatpragmatic's advice: establishing a boundary and taking action yourself works.

I was in your situation. I dealt with it by asking for specific actions, like "please store your bicycle in the bike cage" or "please find time this month to remove the computer tower from the floor."

This met with mixed success.

I had to be ready with my back up action: what I needed to do to have a clean workspace if I had to do it myself.

• Practice non attachment. Don't get too hopeful that the hoarding will cease just because you asked.
• Be ready with an action (moving papers to her desk),and a plan for when you will do it (at the end of the day).
• Focus on the action and how it helps achieve the outcome you want.
• Pay attention to your own feelings over time. Are you growing resentful? Isolated? Feeling ignored? Then it's time for more conversations with your coworker and possibly a manager.

And keep an ear out for opportunities to change office/workstations. Your coworkers likely have sympathy, if they know what your office is like.