Running the sound from my computer out to my big fat stereo. Input to the stereo is Ys with input from TV and computer both. Sound from TV feeds back through the computer speakers when the stereo is all the way down.

Is there a one-way 1/8" cable that will let the computer send but not receive audio signals?

Mac Mini Monterey, if it matters.

Short answer is "no". Cables alone are simply conductors. If the TV and Mac are both running into the same input sockets on the amp, the problem you're having is due to impedance mismatches, and could damage the audio components in the TV and the computer.

An A>B>Y switch box would be a better solution. Or run the outputs from the TV and Mac to separate inputs on the stereo amp.

I use one of these to feed two sources into a pair of powered speakers and switch between them, I think it would work for your application

Nobsound Little Bear MC1022 Mini 2(1)-in-1(2)-Out RCA Stereo Audio Switch Audio Switcher Passive Speaker Manual Selector Splitter Box Audio Sharing https://a.co/d/2j2cmSp

Thought as much. Most of the point is to send audio to the stereo without crossing the room to flip switches which is why they're Y-ed into the same input.

Switchbox would have to be on my desk, meaning cables back and forth more than once.

Ah well. My aging knees will just have to deal with the 12-foot walk.

Thanks @dragondreams and @chinacat

You could use a cheap mixer to combine the signals.