How to configutre specific devices to use a particular ISP in v18

You can do this with Sd-WAN PBR Rules. 

See: community.sophos.com/.../routing-in-xgv18-with-sd-wan-pbr

I actually did that but they could not access the internet.

Let me state all I did.

1. I created their MAC addresses as host list

2. I created a firewall rule then used the source network/device as the host list created.

3. Created a NAT rule(not linked from the firewall rule itself) so that I can select the outboud inteface as the WAN port of the ISP they should route their traffic and also inbound interface as my LAN.

4. I then created an SD-WAN rule incoming interface as my LAN then the source network as the host list created then primary as the ISP I want the users to use and secondary as none.

With all configurations stated above, still no internet access.

You do not need a NAT Rule for outbound. It will be fetched by the Default NAT Rule. 

You only need a SD-WAN Rule to bound the User A to a specific interface. 

Thanks LuCar Toni would check and reconfigure.

Do I still need to set a firewall rule for this?

Do I still need to set a firewall rule for this?