IPv6 and VPN client issue

Hello all,
I have a network with a DHCP server that assigns IPv6 addresses to clients.
Some clients have e VPN Client (Cisco AnyConnect) installed.
I have noticed that when the VPN connection is active on such clients, internet is incredibly slow. This happens probbly because they try to use IPv6, but VPN server drops IPv6 packets and provides IPv4 connection only. If the VPN connection is not active, everything works fine.
If I disable IPv6 DHCP, then connection is fast as expected also when VPN connection is present.

Is it possible to assign IPv4 (and not IPv6) on some clients in the network? Otherwise, how to deal with this?

It would be best solved on the client. Either disable IPv6 on these clients or assign then to an IPv4 only network.