Is it possible to lock versions of packages in Anaconda?

There are two bits to this. First, you can prevent conda from auto-updating by changing the configuration option auto_update_conda to False:

conda config --set auto_update_conda False

The other bit is to pin packages to certain versions. For those packages you don't want to update, you can pin the version by adding a line to a file called pinned (you might need to create it) in the environment's conda-meta directory. The syntax is

[The code] below [placed in conda-meta/pinned] forces NumPy to stay on the 1.7 series, which is any version that starts with 1.7, and forces SciPy to stay at exactly version 0.14.2:

numpy 1.7.*
scipy ==0.14.2

See the documentation for more information.