Without cheating (save game edit/exe modifications) it is impossible.
And even with editing most probably next month you will get event turning you into PD. It is how V/R engine works - Democracies elect new gov every 4 years. Only way to be Democracy without voting rights (per se - it is not a Democracy

) is to dissect exe file, change code and compile it back to exe.
Edit saved game, it is the only solution I can provide here. Perhaps like this:
government = democracy
executive_designation = parliamentarism
party_system = two_party [or multi_party]*
national_value = liberty [or whatever you like]
voting_rights = none
press_rights = censorship
political_parties = all_allowed **
trade_unions = all
public_meetings = yes
* - important becasue with one_party it is certain PD
** - right to ban = con monarchy and none = monarchy/PD
Anyway, I will repeat myself. Probability it will work is next to nothing. V or R have built-in SDASGC [Self-Defense Against Stupid Goverment Configurations]. :rofl: