There are several settings within the RigPi browser that can do that.
In Settings, Advanced Radio Settings, PTT in the lower left should be "None"
Also, for Icom radios, in the lower right, Disable Split Polling should be Checked.
Then in Settings, Keyer, the checkbox for "PTT on when radio connected" should be UN-checked.
Try that and let us know how it works.
73 - Mark - K8MHW
'A RigPi Enthusiast!' - I am not otherwise affiliated with MFJ or RigPi