The King's public prosecutor puts together a list of people who abstained from voting without a valid excuse and they are summoned to court. Depending on how many elections they've skipped, they will either receive a small fine or be ineligible to vote for 10 years and have issues if working for a public authority. This can be contested, and doesn't appear to be enforced these days.
Article 209 and 210 of the Belgian electoral code covers the punishment guidelines:
Art. 209. Within 8 days of the proclamation of the representatives, the public prosecutor of the King puts together a list of the voters who abstained from voting without valuable excuses.
These voters are summoned up by simple reprimand before the criminal court which renders a judgment without appeal, the prosecuting body being heard.
If they are Belgian citizens living abroad and registered in the consular register of the population, the prosecutor of the King hands out the list to the Federal Public Service Foreign Affairs, Foreign Commerce and Development Cooperation. It ensures that the mention of the registration municipality of the voters concerned is eliminated from the consular registry of the population by the diplomatic posts or consulates.
Art. 210. A first arbitrary absence is penalized with a reproval or fine of five to ten euros depending on the circumstances.
In case of recurrence the fine shall be of ten to twenty-five euros.
An additional prison sentence shall not be pronounced.
If the arbitrary absence occurs at least four times over a time period of fifteen years, the voter shall be deleted from the voters’ list for ten years regardless of his/her criminal record. During this time he cannot receive any nomination nor promotion nor distinction from any public authority.
In the cases provided by the present article the suspension of sentence on probation cannot be ordered.
The conviction can be contested over a period of six month after the notification of the judgment. The appeal can be made by a brief statement at the municipal administration free of charge.
It's not clear if the Belgian government's official policy is to enforce this or not, though at least one source claims that no fines have happened since 2003 simply because the public prosecutor hasn't filed any cases. If that is the case, then it seems like the enforcement method is the potential of small fines if enough people begin shirking their voting duty.