You cannot edit l2 .unr files... best case scenario you could extract them and edit them in 3ds max.. but you totally cannot import them back successfully.. at least not without some modified unrealED (nobody is going to do that, trust me) however you could theoretically replace some world areas with blank ones by replacing the .unr file with a blank one.
for example..
take 15_19.unr which is pretty much blank.. there is no terrain in there.
decode it with l2encdec
rename it to (for example) 22_19.unr (contains Oren town)
encode it back with l2encdec
and finaly replace it with the original 22_19.unr
that theoretically would make Oren area disappear leaving a big hole (or something) .. i have of course not tested this, just saying
but.. let me ask this.. 1) why in the world would you wanna do that? :/ no offence buts IMO it's stupid.. and 2) you can make an area that auto-teleport you out of there server side (if you know java)
and also the gracia content tab is hardcoded in the client.. so no you cant remove it as far as i know.