First, let's look at what you absolutely do not want to do about it.
Often, when describing this problem to others, you will get a standard piece of advice: change the spark plug wires to suppression type wire. Don't do it! Suppression ignition wire will defeat the whole magneto system. If you could see the difference between the spark which is delivered to the spark plug using solid core wire versus suppression wire, you would understand. If you spin a magneto on a test bench with solid wire the spark produced is whitish blue like a bolt of lightning with a bluish corona surrounding it. With suppression wire the spark produced is dramatically weaker appearing red with a purplish red corona surrounding it. It's not a little different; it's a lot different. While it is true that you will stop the problem of the interference from the ignition wires bombarding your tachometer and keeping it from working, you will also stop the engine from running well because you will have greatly reduced the intensity of the spark at the spark plug gap. You will have also put such a load on the magneto coil that its lifespan will be greatly reduced. No mater what anyone tells you; don't install suppression wires on a point-triggered magneto.
So, what options does that leave for solving the problem.
Well, first of all, you could of course eliminate the problem by using a mechanical tachometer. Mechanical tachometers are driven by a cable, which attaches to the body of the magneto. If you have a Vertex or Mallory magneto without the tachometer drive, Don Zig Magnetos can add one to your magneto so that you can use a mechanical tach.
However, if you absolutely have to have an electrical tachometer, because you are using it as a memory device or must have it because you are using it in conjunction with other electronic equipment that sets shift points etcetera, then you are going to have to shield the magneto signal converter (or relocate it), the tachometer, and, perhaps most importantly, all the wires going to and from them, from the electronic field created by the magneto and the solid core ignition wires. This can be accomplished by bundling the wires, where possible, and routing them though shielded conduit and then grounding the conduit. In this way the electromagnetism is carried to ground and eliminated as a source of trouble.
Commercial braided shielding is available at good electronic stores and can be used to shroud the magneto signal converter, the tachometer, and its wires. As a temporary makeshift solution, even a simple aluminum foil shield will help suppress the radiation & noise. In extreme cases, shrouding the ignition wires themselves in grounded braiding may be required to eliminate the problem.
An example of a factory engineered shielding system can be seen on early sixties Corvettes. On those cars, the ignition wires were solid wire Packard 440 which, like all solid core wires, were surrounded by an electromagnetic field. Because the distributor was very close to the dash mounted radio and because the car had a fiberglass, rather than a steel firewall [which would have shielded the radio], there was a terrific radio interference problem, which the engineers had to overcome.
The solution that they developed was to create a formed sheet metal shield which fit over the distributor and the ignition wires and which was grounded to the engine block. This shielding solved the problem completely.
Now that you understand the problem, you can rest assured that you can solve it. It may take some persistence and ingenuity, but it can be solved. Every day piston engine equipped aircraft fly around without having any problem. Every aircraft piston engine has two point-triggered magnetos that give off loads of electromagnetic pulsation. They don't have a problem because all of the radios and sophisticated electronic avionics on board the aircraft are shielded. If they can do it, you can do it.