Just trying to work out a few things and wondering how straight forward it would be to completely remove my visonic powermax alarm. It’s hard wired to a fuse spur next to my fuse board.  the landline is hard wired into the unit too  I think. 
I have the installer code. What I’m not sure of is, if I open the control panel and remove the battery backup then turn the power off will the external bell siren be set off. There’s no way I’m going up a ladder to the bell housing. I think it has a backup battery in it. So I would just like to leave it where it is for now. Or can I disconnect the bell housing siren from within the installer mode 

Fully working systems when the external siren is disconnected from the control panel this will trigger the siren.

You may find it doesn't but to decommission a system you would usually get up to the siren.

These things are not designed to be defeated easily.

Agreed you do need to get up to it and remove it

In theory the bell should only sound for 20 minutes and then stop. I would not recommend leaving it indefinitely though these things have a habit of going wrong at the least convenient time. 

That said I would recommend removing it all at the same time, if it hasnt been serviced in a while ho knows what it may or may not do  

In installer mode I can see the siren, what if I delete this from the setup. Won’t that mean once turned off it won’t do anything as it’s removed

TIAS? There is no guarantee a siren with a working battery even if un-enrolled isn't going to trigger at some point in the future.

Ok makes sense I just wasn’t sure if taking it off the system would mean it was out of action but if the battery goes low and could cause interference with stuff then I’ll just get it off. Just wish it wasn’t so bloody high