NHibernate 3.2 comes with its own proxy factory. If you’re using a config file, you just need to remove the proxyfactory configuration property.
I believe the version of Fluent NHibernate that you’re using defaults to use NHibernate.ByteCode.Castle. In that case, you would need to override that setting with the built in NHibernate 3.2 proxy factory:
.ProxyFactoryFactory("NHibernate.Bytecode.DefaultProxyFactoryFactory, NHibernate")