I have seen 2 solutions to this problem.
 
Solution #1 is to assign a limited number of people with Demote rights.  I
personally do not agree with this solution.  In a classic configuration
control environment, once a drawing has been "released" there can never be
any change to it with out a formal change process (ECO/ADCN/DCN/Drawing
revision).  If you follow the tenants of configuration management, no one
should ever be able to change a drawing at a specified rev letter/number
without a revision modifier.  With this in mind I implemented solution #
2.

Solution # 2 give only the Intralink administrator demote rights.  It goes
on to use groups, roles, release levels and folder authorizations to
control modifications to drawings.  It works like this:
Groups are created to ease the assignment of roles.  My groups are 
Design Engineer
Drafter
MFG engineer
Project
Configuration Management

Assign as many groups as you need.  


Several roles are created for the users as follows.
Full
Modify Release
Limited
View only

Full Role:
Users who are trusted to used good practice and have a need for
flexibility are assigned the Role of "Full"  with all object actions
except for Demote, Delete, And the 2 forced checkin options.  they are
Assigned all of the Folder actions except Delete None-versioned objects
and edit folder authorizations.

Modify Release:
Used to allow trusted users the ability to set a new rev letter & release
level. Object actions allowed are ONLY THE FOLLOWING:  set revision,
Promote Object, override RTP & Checkout  Folder Actions are ONLY THE
FOLLOWING: Create non-versioned objects set CS status & View object(this
is critical as it only allows viewing of the object and the setting of a
new release level.)
 
Limited Role:
Users who need some supervision with data management are assigned the Role
of "Limited"  with all object actions except for Demote, Delete, And the 2
forced checkin options.  They are Assigned all of the Folder actions
except Delete None-versioned objects, create branch, edit folder, edit
folder authorizations.

View only:
Object actions: Check out only
Folder Actions: View object only

Release levels:

You can have as many as you like.  I use something like this:
WIP: typical out of the box release level 
Prototype: Used for non production drawings
Released:  Controlled drawings.

With Groups, roles & release levels defined, you tie them all together
using the folder authorization.

When you create a new folder, you assign authorizations by group to the
release levels you have defined.  it might look something like this:
-------------------------------------------------------------------------
|Name         | Folder Role  | WIP      | Prototype      |  Released     
|
-------------------------------------------------------------------------
|Engineer     | Full         | Full     | Modify Release | Modify Release
|
-------------------------------------------------------------------------
|MFG Engineer | Limited      | Limited  | View Only      | View Only     
|
-------------------------------------------------------------------------

You can now add as many groups as you like and set revision control
permissions by folder...

There is a lot more to setting this up than meets the eye, however once
you develop a consistent process it make administration of the database a
very easy.

Hope this helps you,

Roger Ramsey
General Dynamics- OTS