Can you un-enroll from 401k at any time? Or is it like health insurance where you are locked into it until open enrollment periods?
You can stop contributing or change your contribution at any time, but there are some restrictions on taking the money out that you should be aware of. For example, you can't cash out the 401k while you are still employed by the company that hosts the 401k.
Also, even when you can, you will be penalized for withdrawing funds before you turn 59 1/2.
FYI: On insurance you aren't "locked in" so much as you are paying for a fixed amount of time (like a lease on an apartment). You can get out of health insurance when that runs out by just not paying for another period.