Allowing only one tag per post

I found this code in another post:
  # clean string to remove spaces and force lowercase
  def self.clean_string(string)
    (string.downcase).gsub(" ","")
  end
My form:
  <% form_for @post, :html => { :multipart => true } do |f| %>
  #...
    <%= f.label 'Topic' %><br />
    <%= f.text_field :tag_list %>
  #...
  <% end %>
I could use something like this in a callback to remove commas from the
input on the text_field correct?