The Merentha MUDlib allows you to customize how you wish to handle
classes, if at all.

There have been many discussions on how to impliment classes.  Do you
force a player to join a class (or profession) like mage or fighter, or do
you allow them to gain and grow their own skills?  And if you allow
classes, what about multi-classing?

The Merentha MUDlib offers all these possibilities, and it even offers the
ability to use all of them at the same time.  By default a player does not
'join' a class, but instead they practice and train skills.  Once their
skills are high enough they may become a member of a class for which their
skills are good enough.  For example, once you have learnt enough about
the ways of magic they may be able to join as a member of the 'mage'
class, but you do not need to join it.

This system also allows multi-classing, since a player could in theory
become members of all classes by gaining all the skills needed.  Once a
player is considered a member of a class they gain skill bonuses based on
the skill and how many classes they are a member of.

With some minor changes to the lib you could force players to choose a
class at character creation, or limit the number of classes a player could
join.