jQuery Grid Plugin - jqGrid

Hello

I'm returning userdata via an ajax call, for example:

<userdata name="val_a">a</userdata>"

The next call may return:

<userdata name="val_b">b</userdata>"

I then loop through the return user data array. The problem I have is that where I expect to get val_b back on the second call I get both val_a and val_b even though I returned just val_b in the second call. Is there a way to clear down userData?

Many thanks

Matt