Are you using anything as a coolant or lubricant? Soapy water works quite well with rubber.
I have not done a lot of machining on rubber but I did determine that a very sharp tool with a large top rake worked best. Maybe you could try the micrograin carbide endmills with a very high helix angle that are made for aluminum. They do have a very sharp edge and the helix angle effectively gives them a large top rake.
I woul try running at a modest speed, 2000 to 4000 rpm and use a rich soap solution as a coolant.
An open mind is a virtue...so long as all the common sense has not leaked out.