Make readonly or lock Grid in Service Activity

Hi everyone!

I have added a grid in service activity of another entity . I would like that grid to be disabled or readonly so that users are not able to even select  record or add a new record.

I have tried this code but nothing happens.

// Disable a subgrid on a form
function disableSubgrid(subgridName) {
document.getElementById(subgridName + "_span").disabled = "true";
}

I have of course added the name of the grid as an argument but nothing happens. I can still add records to that grid.

Is there a way i could do it?