How was collective farming enforced?

Forced by the state

Here is some evidence: Stalin's 1929 order simply required farmers to pool their land and their equipment, and to work in future under the orders of the collective farm committee (which was under the control of the Communist Party). No other details - such as how the workers would be paid - were given, and in 1930 Stalin even sent a contradictory order that 'small vegetable gardens, dwelling houses, some dairy cattle, small livestock poultry etc. are not socialised' (at which point, many farmers withdrew from the collectives).

Later rules, however, enforced collectivisation, prescribed punishments for 'enemies of the collective farms' (such as the kulaks), stipulated that 90% of the produce had to go to the state (with 10% left to feed the collective), and set up Motor Tractor Stations to provide mechanisation.