Extra-territoriality
Do national regulators enforce rules against entities operating abroad?

Belgium
Yes, this is possible as the respective rules are criminally sanctioned. However, as a general observation, the enforcement risk against foreign operators seems rather low.
An exception to that last statement should be made for prize promotions which fall under the ambit of 'games of chance' (poker games, casino games, etc), which are heavily regulated and enforced by the Belgian Gaming Commission, including through actions against operators established abroad.