Keep in mind that in Ruby, classes and modules are just ordinary objects that happens to be instances of the classes Class and Modules respectively.

So for all practical purposes, if you define a module it is not much different than if you define a class, and then instantiate a single object (it is slightly different in that your object will be an instance of your class rather than of the class Class).

Modules are namespaces for Ruby (and pretty much only differs from classes in that you can't create instances of modules)

What you describe above is done with modules:

    module UserContentSpamChecker
        def is_spam?(content)
            ...
        end
    end

    class MyOther Class
         include UserContentSpamChecker

         def foo
             is_spam?(content)
         end
     end

If you want to be able to alias it, you'd do it with a method:

    module UserContentSpamChecker
        def self.is_spam?(content) # note the "self." to define a method callable on the UserContentSpamChecker object itself (of class Module)
            ...
        end
    end

    class MyOther Class
         def spam_checker; UserContentSpamChecker; end

         def foo
             spam_checker.is_spam?(content)
         end
     end

(or you could do it with a class variable or class instance variable - example class variable:)

     class MyOtherClass
          @@spam_checker = UserContentSpamChecker

          def foo
              @@spam_checker.is_spam?(content)
          end
     end