It depends on the scheduler you are using.
For the fair scheduler which is currently the only supported scheduler in CDH 5 you can achieve it in a number of ways.
If you have queue based on user names then you could set the max applications for the user (mappers and reducers is not a YARN queue thing) in the allocations file:
<user name="forbidden_user"> <maxRunningApps>0</maxRunningApps> </user>
The other solution would be to use a 'reject' as the last rule in the queue placement policy and make sure that the user does not match any of the earlier rules.
If you really want to you could use queue ACLs also.
For all these options see here: http://hadoop.apache.org/docs/r2.3.0/hadoop-yarn/hadoop-yarn-site/FairScheduler.html