There are couple problems. The /interface pptpt-server set… command looks like it needs to be inside the if {} statement, and that same command didn’t have enough information. It needed to know which pptp-server item to choose. The code below finds the pptp-server username that corresponds to the user-manager username, and then it disables it.
:global disabled
:global usrname
/tool user-manager user
:foreach i in=[ /tool user-manager user find customer=brendanM ] do={
:set disabled [get $i disabled]
:if ($disabled = true) do={
:set usrname [get $i name]
/interface pptp-server set number=[/interface pptp-server find where user="pptp-$usrname"] disabled=yes
}
}