Radio and television broadcasters may not become third persons and respectively can't contribute to the campaign expenditures.
Art. 16. "(4) Each mayor may reserve a space for election posters 16 days before the election day. Such space must meet the requirements of equal treatment of all political parties and coalitions standing for election or candidates running for Senate. Communication media of the region or municipality or legal person managed by the region or municipality may not be used for election campaign."
Art. 16e. "(2) For the purpose of this Act, the following entities may not become registered third persons: ... the radio and television broadcasters."
Source: Articles 16(4) and 16e (2), Act No. 247/1995 on Elections to the Parliament of the Czech Republic and on the Amendment of Certain Other Laws, 1995