Prevent HAProxy from toggeling back from fallback to primary

[Claudio M.]

Hi, ive the folowing configuration 

backend web1.mi.ext
        mode http
        option httpchk
        balance roundrobin
        option httplog
        option http-server-close
        cookie CLUSTID insert
        source 0.0.0.0 usesrc clientip
        server web1b.mi 192.168.0.29:80 cookie web1b.mi check port 80 maxconn 
100
        server web1d.mi 192.168.0.21:80 cookie web1d.mi check port 80 maxconn 
100
        server web1.bk  192.168.0.43:80 cookie web1.bk  check port 80 maxconn 
100 backup

where web1.bk is obviously the backup server

I need that when both primary servers go down and web1.bk go online, when 
web1b.mi and/or web1d.mi go up haproxy not switch to these

Is this possible?

Best regards
Claudio