You could set up your destroy method as follows:
def destroy
@comment = Comment.find(params[:id])
if @comment.user == current_user
@comment.destroy
format.html { redirect_to :back, alert: "Comment Successfully destroyed" }
else
format.html { redirect_to :back, alert: 'You can\'t delete this comment.' }
end
end
If you want to allow your admin to delete any comments, you can change
if @comment.user == current_user
to
if @comment.user == current_user || current_user.admin?