Disabling AMM for ASM is sorta tricky. I found an Exadata note on MOS that describes how to do it. I was successful doing this on a test machine.
These steps have been verified on 11.2.0.4.
alter system set sga_target=400M sid='*' scope=spfile;
alter system set pga_aggregate_target=200M sid='*' scope=spfile;
alter system set memory_target=0 sid='*' scope=spfile;
alter system set memory_max_target=0 sid='*' scope=spfile /* required workaround */;
alter system reset memory_max_target sid='*' scope=spfile;
Reference
MOS Doc including steps for disabling AMM for ASM