Hi,

Please refer to the code:

from odoo import models, api

from odoo.exceptions import ValidationError

class AnalyticLine(models.Model):

    _inherit = "account.analytic.line"

    @api.model

    def create(self, vals):

        task_id = vals.get("task_id")

        if task_id:

            task = self.env["project.task"].browse(task_id)

            if task.is_closed:

                raise ValidationError("You cannot log time on a closed task.")

        return super().create(vals)

    def write(self, vals):

        if "task_id" in vals:

            task = self.env["project.task"].browse(vals["task_id"])

            if task.is_closed:

                raise ValidationError("You cannot reassign timesheet to a closed task.")

        return super().write(vals)

Hope it helps.

On the Settings tab of the Project, uncheck "Timesheets - Log time on tasks" to prevent further timesheeting: