Partial water change outs can reduce the amount of anything dissolved in the water but not totally remove it. Although it is sometimes necessary, draining and refilling a pond should only be used as a last resort!
Drain some water from the pond before refilling; ideally pumped or siphoned from the dirtiest conditions from the bottom of the pond.
Remove no more the 20% of the pond volume at a time; 5 to 10 percent of a pond's water per week is more in line. Larger water changes are likely to upset the biological balance of the pond.
A water change out reduces the amount of a substance in the water by the same amount as the percentage of water replaced. Remember the concentrations of anything beneficial in the pond is being reduced at the same rate!
It is very common for pond keepers to skip making these routine water change outs. This is not advisable as many things can build up in your pond over time and this is the only way to reduce them. Experienced pond keepers know that their fish are healthier and stronger when these water change outs are conducted.