Degrading the cabling will never guarantee that the link will operate at 10Mbps. The endpoints transmit their capabilities in the regular fast link pulses and pick the best available speed and duplex option both are capable of, see https://en.wikipedia.org/wiki/Autonegotiation.
Depending on the ethernet driver software the outcome when the cabling does not support the negotiated speed the link may either just fail to operate due to packet corruption, or the driver may opt to advertise slower speeds and renegotiate when significant errors are detected.
If inserting a 10Mbps switch is not an option the only reliable method would be to construct something to remove the fast link pulse signalling, but still retain a single link integrity test pulse. This is likely to be more difficult than inserting a switch.