Yes, you can control how high new players in your club are allowed to self-assess their own level.
This rule applies to players who have selected your club as their home club in the app and have not yet set a level. When this rule is active, they will be required to choose a level that is no higher than the maximum you have defined.
You can configure this setting in Club Admin -> Settings -> Rules & Limits -> Match settings (at the bottom of the page).
The player will be notified in the app that your club has a maximum self-assessment rule, and will see the limit you have set.