/ip route set \[/ip route find\ gateway=xxx.xxx.xxx.xxx\] disabled=yes
/ip route set \[/ip route find\gateway=yyy.yyy.yyy.yyy\] disabled=no
/ip route set \[/ip route find\ gateway=yyy.yyy.yyy.yyy\] disabled=yes
/ip route set \[/ip route find\gateway=xxx.xxx.xxx.xxx\] disabled=no