Some ideas on how to block a wheel on a metallic rod?
Use a hacksaw (or just a hacksaw blade if you don't own a hacksaw) to make a groove all around the rod. If you had a small electric grinder (Dremel or similar) that would also be an option, but the hacksaw will work and is a hand tool.

Use an "E-clip" in the groove (perhaps with a washer towards the wheel, first.)