To disable the Send with Conga Sign feature in Negotiate mode, add the disableSendWithCongaSign=true parameter to your button code as seen in the following example:

{!URLFOR($Action.Contract.APXT_Redlining__View_Redlines, null, [altId=Contract.Id, disableOpenInWord=true, disableNegotiate=false, disableTrueUp=false, disableSavePdf=true, disableSendwithCongaSign=true, enableRecursiveMergeSync=true], false)}

To enable this feature again, change the parameter to: disableSendWithCongaSign=false.