Use a cheap mains noise filter
an expensive super separation transformer ?
An "Ultra Isolation Transformer" has minimal coupling capacitance between the primairy and secondary windings, and the iron of the core damps transfer of higher frequencies than 50 Hz. Resulting in minimal import of noises.
But the high price of such transformers (1800VA = $835) can be a reason not to buy it, as the netto effect is unknown before its use in the specific situation.
A test using a good mains noise filter (Schaffner 10A = 23,52E) is more attractive. No cure? Less pay.
In both cases, a dedicated ground connection, noise free, from mains safety ground separated, is a necessity.
I inserted a relative cheap 10 Amp mains noise filter (Chaffner FN2060-10) between the supplying mains power group, and the power lines to all my radio- and computer equipment. I used the grounded side of the filter as OUTPUT. The side without GND connection is INPUT. The GND connection of the filter was connected to the best "ground" i could find : the steel reinforcement of the concrete walls of the apartment building. The filter was NOT connected to the with noises polluted mains safety ground.
This way, i am cheaply and fully separated from the mains supply net for all short wave RF noises.