Remove custom user property

Are you using user profiles?

yes

And the property you are deleteing is in the profile or on the site collection user list? If on the user list then check if the same property is defined in the profiles and has the "replicable" atribute selected. If so, then during the synchronization this property will be recreated on site collection and values from the profile will be copied to this property on site collection.

In the site collection user list...

I've have deleted the fields from the user properties  - so I can't remove the "replicable" checkmark...  

You can now delete it in the user list properties - when you delete the property in the user profiles it will not be deleted in site collections - you need to do it manually.

how do I delete it in the user list properties?

Open page "/_layouts/people.aspx" adding your site collection address - for ex http://intranet/_layouts/people.aspx or http://intranet/sites/sitecollection1/_layouts/people.aspx

On this page click on the "Settings" in the bar above the user lists and then choose "List settings". There you will be able to delete the column.

Nope... that's not possible..

Is the column not visible there or the column does not have the "Delete" button?

Column not visible...

It's at the  _layouts/userdisp.aspx page where the columns was present..

I've now installed Sharepoint manager on the WFE... and by that I can navigate to and delete the fields...but it should be do'able otherwise...

But is it visible on the user list's settings page ("/_layouts/listedit.aspx?List={GUDI}")? You can enter it from the "/_layouts/people.aspx" page?
First open the settings menu and choose "list settings":
 Next scroll down to the column list - it should be visible there:
 

got it removed