Minneapolis--Martin/Williams, part of the Omnicom Group, on Thursday laid off 22 people, or 7% of its staff, citing the slackening economy. The reductions cut across all lines of business. Steve Collins, president of Martin/Williams, said most of his clients had reduced spending and that the company had hoped new business would forestall layoffs. This is the second round of layoffs for Martin/Williams. In April, 18 people were laid off.