But fulltime is a contract thing (at least here) and defined by 40 hrs a week. In my country 32-36 in contracts is also called fulltime. So after those hours, I did my fulltime and now you do not own me until the next 40 hours. Unless working for competitors currently here you cannot make valid contracts to prevent it either.
There are contractual terms, including things that are likely to be conflict of interests or impact performance. And depending on jurisdiction there are also laws on working hours: 48 hours max. per week on average in the UK and EU across all jobs (it is possible to opt out) and with minimum rest times. Because employers can be held liable, if they find out they won't let you.
The comment I was replying to does not make sense.
However, if you are in the EU, then all your employers are jointly responsible for ensuring that your collective working hours don't breach the Working Time Directive, which means 48 hours as the maximum average working week, calculated over a 4-month period, across all employers (excluding certain statutory roles like seamen, law enforcement, and military).