Hello.
You can delete it from
\CMSModules\Forums\Controls\Layouts\Tree\Threads.ascxor
\CMSModules\Forums\Controls\Layouts\Flat\Thread.ascx file, according to what mode (tree vs. flat) you are using.
E.g. on
\CMSModules\Forums\Controls\Layouts\Flat\Thread.ascx it is defined like this:
<%# IFCompare(IsAvailable(Container.DataItem, ForumActionType.IsAnswer), "<div style=\"float: right;\">" + ResHelper.GetString("Forums_WebInterface_ForumPost.Washelpful"), "")%>
Best Regards,
Radek Macalik