The way to clear out an existing field in SL is using the argument JHANull ="true". So, for example, if you were running an AcctMod command, and you are trying to clear out the SerChgWavRsnCode field because it already has something in it, that one line in your AcctMod call would look like this:

<SerChgWavRsnCode JHANull="true"></SerChgWavRsnCode>

The argument is placed inside the first element and nothing is placed where you would normally put your value (between the two XML elements). This JHANull argument can be used for many fields (but not for fields that have a required value).