How to forbid internet access on a specific VLAN

Hello,

first of all, I'm all a newbie on networking stuff, I only know the basics of networks and no further.

I have a HP 1810G-24 model managable switch. 

All I need to achieve is to prevent some of the computers that are connected to switch (let's say ports 20,21,22,23 and 24) from accessing the broadband internet (thru the modem connected to port 1 on switch) but still have access to the rest of the local area network, even those that are connected to ports 2 to 19.

In order to simplify, I have these questions:

1. Is that possible with HP 1810G-24 Switch? If so...

2. Do I need to create 2 different VLANs with different subnet masks, right ? 

3. Can anyone give me a small information on how-to ? 

Thanks by now...