To disable call waiting on MacOS 9 with Remote Access:
Open Remote Access
Edit menu -> User Modes
Select Advanced, click OK
RemoteAccess menu -> DialAssist
Prefix button in lower right
Click Add
Type in "Disable Call Waiting - *70" in the Label field and *70 in the Number field
Click OK, then OK again. This takes you back to the DialAssist screen. Where it says "Prefix: None", click the up/down arrow on the right then choose Disable Call Waiting - *70