Log out of the account and then go to iChat preferences. Click on the accounts tab and then find the account you want to not log in and select it. There should be a checkbox that says "Log in to this account when iChat starts" or something along those lines. Uncheck it and you're good to go.
Log out of the account and then go to iChat preferences. Click on the accounts tab and then find the account you want to not log in and select it. There should be a checkbox that says "Log in to this account when iChat starts" or something along those lines. Uncheck it and you're good to go.