Can a 1:1 transformer prevent the SMPS noise of one load from reaching other loads plugged into a mains outlet?
It's my understanding that a load which has a SMPS can put noise back onto the power cord, which could reach another component that's plugged into the same outlet.


Yes, that is true, but if the device conforms to current EMi legislation, this noise will be quite low. If it's a chinese $1 USB charger, maybe not. Never use a chinese $1 USB charger while in your bathtub, by the way. I've found this helpful to stay alive.


  Is it true that a 1:1 transformer can prevent SMPS noise of one load from polluting the power that goes to other loads?


A transformer has a lowpass effect due to its inductance, so it will filter out some of the HF noise, if it is differential. As for common mode noise, that would depend on transformer interwinding capacitance.

So you'd want a rather bad transformer, with lots of leakage inductance, and low interwinding capacitance (ie, not a toroid).

As Tony Stewart. EE since '75 hints, a transformer is an expensive,  clumsy, and ineffective way to do this. A much  better way would be to:


Increase common mode impedance, with a common mode choke
Filter out differential noise with a LC filter


This is basically what a IEC mains filter does.



But the best is to increase noise rejection in the sensitive equipment.