I have a problem with visitors attending private meetings & discussions in our office facilities

I have a problem with visitors attending private meetings & discussions in our office facilities. Meetings are disturbed by phones ringing & visitors actively making private calls. On their mobile phones .The facilities are open plan some 20 x 10m.

Do you have a jammer for sale that would block incoming & outgoing calls for the area.I would like to give our security officer a unit that he could activate in his pocket & be powered by batteries. Please advise regarding a reliable low cost unit.