How can I eliminate one 3-way switch to leave just one switch on the lighting circuit it's on?
Your circuit contains two 3-way switches. Referring to the diagram below:


if the switch with the incoming "hot" is the one you want single control from (shown at left)-


1) replace that switch with a single-pole switch

2) connect either of the "traveler" wires to the load side

3) remove the other (unwanted) switch and connect the traveler wire used in step-2 to the "switched hot" wire that goes to the lights.

4) the unused "traveler" wire is abandoned




If the switch without the incoming "hot" is the one you want single control from (shown at right)-


1) remove the switch with the incoming hot (shown at left) and connect the incoming hot to either of the "traveler" wires

2) replace the other switch (shown at right) with a single-pole switch

3) connect the "traveler" wire used in step 1 to one side of the switch

4) connect the "switched hot" that goes to the lights to the other side of the switch.

5) the unused "traveler" wire is abandoned