The opposite of disable is enable, so csrutil enable

The manual page and/or self-displayed help can get you this information if you wish to check this answer.

$ csrutil
usage: csrutil <command>
Modify the System Integrity Protection configuration. All configuration changes apply to the entire machine.
Available commands:

    clear
        Clear the existing configuration. Only available in Recovery OS.
    disable
        Disable the protection on the machine. Only available in Recovery OS.
    enable
        Enable the protection on the machine. Only available in Recovery OS.
    status
        Display the current configuration.

    netboot
        add <address>
            Insert a new IPv4 address in the list of allowed NetBoot sources.
        list
            Print the list of allowed NetBoot sources.
        remove <address>
            Remove an IPv4 address from the list of allowed NetBoot sources.
$

The opposite of disable is enable, so: csrutil enable

At the present time there is no manual page for csrutil i.e. man csrutil doesn't provide any help, however executing the command without an argument displays its internal help as shown below.

$ csrutil
usage: csrutil <command>
Modify the System Integrity Protection configuration. All configuration changes apply to the entire machine.
Available commands:

    clear
        Clear the existing configuration. Only available in Recovery OS.
    disable
        Disable the protection on the machine. Only available in Recovery OS.
    enable
        Enable the protection on the machine. Only available in Recovery OS.
    status
        Display the current configuration.

    netboot
        add <address>
            Insert a new IPv4 address in the list of allowed NetBoot sources.
        list
            Print the list of allowed NetBoot sources.
        remove <address>
            Remove an IPv4 address from the list of allowed NetBoot sources.
$

$ csrutil
usage: csrutil <command>
Modify the System Integrity Protection configuration. All configuration changes apply to the entire machine.
Available commands:

    clear
        Clear the existing configuration. Only available in Recovery OS.
    disable
        Disable the protection on the machine. Only available in Recovery OS.
    enable
        Enable the protection on the machine. Only available in Recovery OS.
    status
        Display the current configuration.

    netboot
        add <address>
            Insert a new IPv4 address in the list of allowed NetBoot sources.
        list
            Print the list of allowed NetBoot sources.
        remove <address>
            Remove an IPv4 address from the list of allowed NetBoot sources.
$

The opposite of disable is enable, so csrutil enable

The manual page and/or self-displayed help can get you this information if you wish to check this answer.

$ csrutil
usage: csrutil <command>
Modify the System Integrity Protection configuration. All configuration changes apply to the entire machine.
Available commands:

    clear
        Clear the existing configuration. Only available in Recovery OS.
    disable
        Disable the protection on the machine. Only available in Recovery OS.
    enable
        Enable the protection on the machine. Only available in Recovery OS.
    status
        Display the current configuration.

    netboot
        add <address>
            Insert a new IPv4 address in the list of allowed NetBoot sources.
        list
            Print the list of allowed NetBoot sources.
        remove <address>
            Remove an IPv4 address from the list of allowed NetBoot sources.
$