Block certain user groups posting with certain Tasks

I don’t believe this is possible with restrictions like access rights or row-level security.

You could do this via customisation something like this would work:

public static bool IsUserRole(string roleName, PXGraph graph)
{
    string usrName = graph.Accessinfo.UserName;

    UsersInRoles assignedRoles = SelectFrom<UsersInRoles>.
        Where<UsersInRoles.username.IsEqual<P.AsString>.
        And<UsersInRoles.rolename.IsEqual<P.AsString>>>.View.Select(graph, usrName, roleName);
    if (assignedRoles == null)
        return false;

    else return true;
}

//I am not 100% sure of the field name here.
protected virtual void _(Events.FieldUpdated<CRActivity.category> e, PXFieldUpdated b)
{
    CRActivity row = e.Row;
    if (row == null)
        return;

    if (IsUserRole("someUserRole", Base))
        return;
    else
        e.Cache.RaiseExceptionHandling<CRActivity.category>(row, new PXSetPropertyException<CRActivity.category>("Priority is required"));
}