how to damage Ethernet wire without cutting?

Put a thin coat of something, like epoxy or super glue, on one or both ends of the cable, over the contacts. The idea is to make it so that thet wires cannot make contact with the network card. If you put a really thin coat, it would be difficult without careful inspection that something has been done. You could also put a thin cut of tape over each of the contacts as well. if you do it to both ends, you increase your chances of damaging the wire enough that it will not transmit data. Finger nail polish would also probably work very well.

Ethernet, coax, heck ANY cable can be damaged and lose it ability to transmit, conduct or make magic, by either crimping it or stressing it. Try stepping on one enuff times! I saw one lose 1/2 its transmit dut ot a Kenmore vacuum cleaner. Didn't cut it, but mangled the fool outa it!

If you bend it enough it could break the cabling inside and snap the prongs on the ends.

What sort of project is it?

i get no projects with destruction of objects! i get osmosis in potatos.

Bending the prongs, crushing the prongs, or using electricity, carefull this one could also break you.

I would use staples. Lots of staples. Then pull them out. You'll get interference and a bad connection. Lots of back connection

Bend the prongs.

You could try putting a small nail through it, causing interference and back connection.

break the tip of it?

bend it?