Stimulus artifact while performing P300 experiment

Hello,
I had previously asked some questions about connecting external triggers from a Raspberry Pi 400 on which I have devised a three-event P300 experiment. I have also designed triggers to appear on the OpenBCI, for which you suggested to interface an optoisolation unit as a part of the interface for external triggers. I have fixed the issues with the optoisolation circuit and I am now getting triggers on my OpenBCI now. I am attempting to conduct a P300 experiment for a total duration of 10 minutes.
While doing so, I have observed that there are stimulus artifacts (Digital triggers) that bleed into the EEG signal data. Attaching an image for your reference. Any advice on overcoming these would be appreciated. Thanks.