Dollar Tree is planning to lock up some of its merchandise and may discontinue the sale of certain products, as theft makes a dent in the company’s profit margins. Richard Dreiling, Dollar Tree’s CEO, said on a call with analysts Thursday that the company is introducing new measures in the second half of the year to combat “shrink,” or the loss of inventory from theft and other causes. “We are now taking a very defensive approach to shrink,” Dreiling said, noting that such inventory losses “advanced a little further than what we had anticipated.” Dollar Tree and Family Dollar, both...