You will need to edit the widget_statistic module template to accomplish this.
Find :
Code:
{vb:phrase active_members_x, {vb:number {vb:var statistics.activeMembers}}}
in the widget_statistics module and replace with
Code:
<vb:comment>{vb:phrase active_members_x, {vb:number {vb:var statistics.activeMembers}}}<vb:comment>
This modification may not survive and upgrade so you might have to do the edit every time you upgrade.