What is 'calcium conductance'?
Conductance is the inverse of resistance, and measures how much of a given substance flows throught a channel. In this context, it means how many calcium ions enter the cell in a period of time.

There are at least two ways potassium channels may prevent the calcium to enter in the cell.

1) Potassium intake by ion channels decrease the membrane potential, restoring it to its rest state. Since many calcium channels are voltage-dependent, a reduction of the membrane potential would close them, effectively decreasing the calcium conductance.

2) Potassium channels may be coupled to different signaling pathways (i.e. G proteins), wich may affect indirectly other calcium channels.

In the case of opioid receptors it seems to be the first mechanism. With potassium channels open, the neuron is less likely to be activated, because it needs higher stimulation to reach the action potential. Calcium channels open in neurons mainly during action potential events (althought they may exist other calcium channels that open in other conditions, i. e in response to hormones or neuromodulators).