Using route-map, you can control in/outbound BGP announcement and apply various attributes :

router bgp 65535
network z.z.z.z
neighbor y.y.y.y remote-as 65500
neighbor y.y.y.y route-map ISP-out out
!
route-map ISP-out permit 10
match ip address 100
!
access-list 100 permit z.z.z.0 0.0.0.255

Recommended to use ip prefix-list since it is lower on cpu/memory resources :

ip prefix-list IPV4-OUT seq 5 permit z.z.z.0/21
ip prefix-list IPV4-OUT seq 10 deny 0.0.0.0/0 le 32
!
route-map ISP-out permit 10
match ip address IPV4-OUT
!
neighbor y.y.y.y route-map ISP-out out