My main computer is an oldie iBook (OS 10.4.11) with Claris Emailer being replaced by Mail to read <> mail messages via POP mail. It works fine

HOWEVER, on the home computer (a newer iMac (OS 10.5.8), I have accidentally goofed in august of last year (dunno how) and I have today on it TWO MobileMe IMAP accounts opened on Mail on the same <> account. Both accounts ARE activated at the same time and effectively block each other. Whenever I attempt to do a change on either one of these two accounts, an error message specifies that "the other" account uses the path to _myAccount_/Library/Mail/Mac-_MyAccount. Since I don't need these messages and stuff (everything I care about is on my iBook), I wand to blast the damm thing with dynamite. I have already deleted the file located in My account library in Mail. No result as each of the two accounts simply asks for my password, recreate the single file upon restart and both compete again on the single file with dual access. Needs more serious dynamite . Any idea ?