I figured out how to get it to work with REST but come to realize that I need to pass to the service a list instead of a single username to bulkify the process.  I can't seem to figure out how to pass a list instead of a single 1 username.

I can't seem to find a complete simple example that shows Apex calling a custom apex REST service with a list as the parameter to the service.

This is the working callout as a single value...

public class SandboxManagerCallout {
  @future(callout=true)
    public static void DeactivateUser(String NamedCredential, String UserName) {
        // The line below is only here to save time during testing by reactivating users.  
        // This should always be set to FALSE in PRODUCTION.
        Boolean isActive = true; 
    String EndPoint = 'callout:' + NamedCredential + '/services/apexrest/SandboxManagerAPI/' + UserName + '/' + isActive;        
        HttpRequest req = new HttpRequest();        
        req.setEndpoint(EndPoint);
        req.setMethod('POST');
        Http http = new Http();
        req.setHeader('Content-Type', 'application/json;charset=UTF-8');
        //req.setBody('{"UserName":UserName, "isActive":isActive}');
        req.setBody('{}');
        HTTPResponse res = http.send(req);
        System.debug('---Response---');
        System.debug(res.getBody());
    }
}




AND THIS IS THE WORKING SERVICE:
@RestResource(urlMapping='/SandboxManagerAPI/*')
global with sharing class SandboxManagerAPI {  
    
    @HttpPost
    global static void doDeactivate() { 
        Integer USER_NAME = 2;
        Integer IS_ACTIVE = 3; 
        System.debug('---DeactivateUser.doDeactivate()---');
        RestRequest req = RestContext.request;
        RestResponse res = RestContext.response;
        System.debug('req: ' + req.requestURI);
        List<String> Parms = req.requestURI.split('/');
        String UserName = parms[USER_NAME];
        String isActive = parms[IS_ACTIVE];
        //String userName = req.requestURI.substring(req.requestURI.lastIndexOf('/')+1);      
        System.debug('Username Passed In: ' + UserName);
        System.debug('isACtive passwed In: ' + isActive);
        User user = [SELECT Id, Username, isActive 
                     FROM User 
                     WHERE userName = :UserName LIMIT 1];
        if (user != null) {
           //user.isActive = false;  //make sure to uncomment this line and comment the one below after testing is complete           
            user.IsActive = Boolean.valueOf(isActive); //this line is only for testing
            Try{        
                update user;
            } catch (System.exception e) {

            }
        } else {
            //Could not find the record so return a message that 
            //informs the caller that the record was not found
            
        }
    } 
}