This depends on the laws restricting this government and whether the government has powers that allow it to freely change the law.
If making statements that did not cohere with the reigning political ideology classed as treachery, punishments with which the astronaut could legally be threatened might include demotion or the loss of her job, imprisonment, torture, execution or the similar mistreatment of friends or family.
The efficacy of these legal threats in preventing undesirable proclamations would also depend on whether the astronaut's trip came with a return ticket. Will she ever need to go back to Earth and face the consequences? And does she have friends or family she values highly enough to overcome her political motivations?
By screening out applicants with tendencies towards contrarian thinking, non-empathic personality traits or weak social roots on Earth, you could dramatically reduce the possibility of your astronaut departing from the dominant political narrative when taking their first steps on a new world.
If stronger coercive methods were legal for your government they might also make use of torture and propaganda techniques to decrease the likelihood of the astronaut even thinking contrarian thoughts. For fictional references see the Two Minute Hate in Nineteen Eighty-Four or Alex's indoctrination in A Clockwork Orange (I'll sidestep the controversy of mentioning real-world examples here).
A technologically advanced society could also conceivably intercept or inhibit the formation of certain thoughts (or their expression in speech) through cybernetic or pharmaceutical interventions.
As explained in Michael Kjörling's answer, the government could and likely would also ensure that the astronaut's means of transmitting messages back home used encryption rendering transmissions unreadable for anyone without the necessary decryption algorithm. The government could then process and edit these transmissions before making them available to the general public. If real-time transmission to the public were necessary then some sort of word filter mesh (perhaps used in conjunction with cybernetics or a simple heart-rate monitor to judge the astronaut's state of mind) could be used to initiate temporary disruptions or scrambling of transmissions, to be replaced later with studio- or computer-generated sound or footage.
Threats in terms of delivering bodily harm directly to the astronaut could also be accomplished via remote controlled systems (here I'm assuming the astronaut has gotten to her current location in a giant, computer-controlled machine the government built). For instance, home base might threaten to open the airlock if the astronaut fails to read from the official script, or assign a weaponized robot to watch her every move.
Depending how theoretical or futuristic you're willing to go the possibilities are endless. The government might sidestep the issue entirely by having the entire landing ceremony carried out by an android.