I have a pol with data file loaded with map1 amd map2.
If I remove a map, POL can't start.
It is possible start and ignore items or npc, and move player to map0 if a map is removed?

i pass on the first question, to the second you can just start the server with 2 maps, move everything to map1 and then restart with only 1 map

You cannot do that only by moving a player. POL loads the datafile, then checks for the map. All I can say to do is use a logon-script to move a player to a certain location in _DEFAULT_REALM if they are in the deleted realm; then wipe all the items and NPC's in that realm manually or via a script. After there are no references in your datafiles to that realm, THEN you can delete the realm and map files. Until then, you will get the error.

POL needs the britannia map, no matter what you set your _DEFAULT_REALM to in uo.em.

I play entirely on britannia_alt (for the pushthrough), but you still need to have at least britannia in the pol\realm directory. Therefore I have 2 map sets loaded - britannia and britannia_alt.