Bad device on network choked out router

I have a BR1 using three vlans. 1st Vlan - Runs good business devices 2nd VLAN - Employees VLAN so they have internet access and to separate them from business. 3rd VLAN - Guest network, throttled down to 3MB

One of the employees unknowingly connected to the guest Network and started running some music streaming apps on VLAN 3.

The next thing you know we couldn't and we couldn't even do a credit card transaction using the devices on VLAN1 , failing with a time out. Ran a speed test from VLAN and was getting responses in Kb (not MB)

Luckily they were only a few people on the network so I started taking them off one by one and found the offending device.

Couple questions:

1. I thought having VLAN's would stop this problem or not allow this to occur especially with the guest network throttled down. But it did affect transactions on another VLAN. 2.4Ghz on guest
2. How would you troubleshoot this and not having to do the trial and error deductive reasoning approach? what tools could we use to isolate one of the devices bad on the network?