Currently, there is no way to disable the DHCP relay agent apart from deleting the entire config tree. It would be nice to have a 'disable' option to be able to disable the service through transition scripts for VRRP backup servers.
There is a PR (https://github.com/vyos/vyos-1x/pull/1852) which tries to accomplish the same thing, but limited to VRRP. I think a 'disable' option is an approach that's more general and easier to implement.