Some caffeine-induced ideas...
That's the great thing about this forum - you learn about a new part of the framework every day!
Anyway, one idea would be to use a separate bean definition file just for your test cases. This test-only file could either leave out the BeanNameAutoProxyCreator completely, or (if that wasn't feasible) change its "beanNames" property not to include the beans you didn't want proxied.
Another idea is to change your existing bean definition file to read the names of the to-be-decorated beans from a system property. In production you would set this property to its current "*Bean" value, whereas in your tests you could set it to whatever you wanted.
I've had another idea (on a roll, just had my coffee!). If your test case is application context aware (or can be made so), and if the advice is only applied at the time of getting the target bean (as opposed to when the app context loads), which may be enforceable via the lazy-init parameter, then your test case could (1) set the proxy creator's "beanNames" not to include "myBean", then (2) get "myBean", which would therefore not be proxied, then (3) set the proxy creator's "beanNames" back to its normal value.
All of the above begs the question of what you mean by "mock" though. You didn't explain exactly what you want to mock, but if you want to mock "myBean", why not use EasyMock or a similar mock-generating tool?
"Now is the EJB of our discontent made glorious Spring"