just make an ip address list of the users that you want to disable, and then a firewall rule that does `action=drop` on this list.
you will then have to make a very simple script - one that disables or enabled this firewall rule at a certain time.
Make address list:
/ip firewall address-list add list=drop_traffic address=192.0.34.166/32
Make drop rule:
/ip firewall filter add action=drop chain=forward src-address-list=drop_traffic comment=DROPME
Make script:
system script> add name=drop_users source=/ip firewall filter enable DROPME
Make scheduler rule:
system scheduler> add interval=24h name="drop_users" \
\... start-time=9:00:00 on-event=drop_users