Ok this is quite simple to make but i was making it for my own use and then thought i might as well put it on here,
i made it quite quick but it works for me fine, feel free to change or modify it in anyway you like.
Any suggestions on improving it would be cool as well.

Here is how to use it;
alt + r -> paste into remotes
New remotes page.
Make sure you paste it "as it is" - don't change the way its positioned around or it may not work because of the two "on open" lines.

When somebody PMs you it will tell you they have messaged you and ask you if you would like to accept it or decline it.
Type /accept "there name" to accept it and allow them to pm you while the PM window is open.

Type /decline "there nick" to decline any more PMs, if they then carry on PMing you it will tell them they are ignored and it will from then on completely ignore them and you wont even see them trying to pm you.
UPDATE: You can now delete a person from your no.pms or msg.declined var using /delete NICK <--- Thanks to Manpreet for the pointer on that.

Update 16/02/2013 Adjusted due to comments on "/dec" being used by mIRC. Thanks to Yawhatnever for pointing this out.

##
#PM block or accept script
#By n_y_f_i/k0nv1kt3d
#I have tested this but if any bugs/errors are found please leave a #comment here.
##
on ^*:OPEN:?: {
  if (%no.more.pm [ $+ [ $nick ] ] == no1) {
    halt
  }
}
on *:OPEN:?: {  
  if (%message.declined [ $+ [ $nick ] ] == no2) {
    msg $nick 14You have been asked 4NOT14 to PM me after having your PM declined, you are now on my ignore list. I will not recive any more PM's from you.
    close -m $nick
    set %no.more.pm [ $+ [ $nick ] ] no1
    halt
    else
  }
  msg $nick 14Please wait for your message to be 4accepted14 or 4declined14.
  echo -a 06/07!0611MESSAGE06/07!0604 $nick 14has messaged you saying:4 $1- $+ 14, Type /accept $nick or /decline $nick $+ .
  set %message.waiting $nick
}
alias accept {
  if (%message.waiting != $null) {
    msg %message.waiting 14Message has been 4accepted14.
    unset %message.waiting  
  }
}
alias decline {
  if (%message.waiting != $null) {
    msg %message.waiting 14Message has been4 declined.
    timermss 1 1 msg %message.waiting 14This means you either have not asked to pm me or you do not have permission to.
    timermsf 1 3 msg %message.waiting 14If you continue to PM me without permission you will automatically be put on my ignore list.
    set %message.declined [ $+ [ %message.waiting ] ] no2
    close -m %message.waiting    
    unset %message.waiting  
  }
}
alias delete {
  if ($1 == $null) {
    echo -a 14Please enter a name you delete from your ignore list if there.
    halt
    else
  }
  if (%no.more.pm [ $+ [ $1 ] ] == $null && %message.declined [ $+ [ $1 ] ] == $null) { HALT }
  else { 
    unset %no.more.pm [ $+ [ $1 ] ]
    unset %message.declined [ $+ [ $1 ] ]
    echo -a 14The nick "04 $+ $1 $+ 14" has been deleted from your ignore list.
  }
}