1. You need a gold sink
Some way of getting gold out of the economy.
Video games have a very similar situation to yours and some of them such as RuneScape have created degradable items that must be repaired by NPCs at a high cost in gold coins as a partial solution to this issue.
A real-world analogy to a game developer would be a government. Hence my next point...
A progressive tax is a tax in which the tax rate increases as the taxable amount increases.
Despite having an endless supply of gold available, monster slayers might stop the hunt when they can only keep 20% of the harvest and decide to go on vacation instead.
The king in power is a hoarder, and keeps all the taxed gold in a large chamber for him to gaze upon. The taxed gold is effectively taken out of the economy.
Loss at sea
The most profitable monsters live on islands far away, and your monster slayers have gotten greedy. As they fill the ships beyond capacity with gold to voyage back home, more and more of them start to sink to the bottom of the ocean.
Another possibility is loss at sea through trade. As one king becomes much richer than another far away that might not have as many monsters, he realizes that the purchasing power of gold in other kingdoms is much higher. An increase in sea travel leads to an increase in lost ships.
2. Limiting factors
You may also have ways of limiting the amount of gold that gets introduced to the economy. Some of these may be:
Lack of good monster slayers
Slaying dragons cannot necessarily be self-taught. And since it is such a profitable venture, the few who know how to do it do not want to waste their time teaching others.
Since very few experienced slayers want to take time off from slaying dragons to teach younger adventurers how to do it effectively, the majority who attempt to follow this career path are inexperienced and unsupported. They end up dying in the first few months to a year.
Even though there is a theoretically infinite amount of gold to be obtained, very few people are actively hunting for it.
People for Ethical Treatment of Monsters (PETM)
Activist groups rally and lobby against the senseless mass killings of monsters and your greed. They slow down your efforts.
Natural Resources Ministries
The king does not want its population to become wealthier than he is, and puts a limit on the amount of monsters each person may kill. A person may need to purchase a permit to hunt specific monsters and may have a limit on the amount that can be killed per year/season.
3. Economic reasons
This isn't my field of expertise and other answers cover the subject, but here's an idea:
Even if the amount of gold in the economy increases, if the amount of gold each person has remains stable, the purchasing power of gold also remains stable.