Modifying SIP VIA header

First of all, string ";received=xxx" was added by the server behind F5 box, NOT Bigip.

Certainly, Bigip can help you remove that string in the response, the basic idea is to find the occurrence in UDP payload and remove it, an example:

rule xxx {
    when SERVER_DATA {
        if {[regexp -indices {(;received=[0-9.]+)} [UDP::payload] location] == 1} {
            set offset [lindex $location 0]
            set len [expr [lindex $location 1] - [lindex $location 0] + 1]
            UDP::payload replace $offset $len ""
         }
    }
}

You should be able to find more in iRULE doc