How to limit the amount of User Points users get?
I found a solution for my question; maybe it can help others too. I actually created a 'correction' Rule that's activated after a user was awarded userpoints and a 'Content' node was flagged with 'Publish Content and grant points to user'. This is how I set up the Rule:

EVENT


User was awarded userpoints
A node has been flagged, under 'Publish Content and grant points to user'


Then, I checked whether the amount of points is higher than 5.

CONDITIONS


Data comparison: [userpoints-transaction:points] is greater than 5


Finally, I calculated the difference between the added userpoints and 5 and substracted this value from the user's userpoints.

ACTIONS


Calculate a value: [userpoints-transaction:points] "-" 5 (variable name: difference)
Calculate a value: [difference] "*" -1 (variable name: substract)
Grant points to a user: grant [userpoints-transaction:user] substract points


I had to insert the second calculation, because otherwise, I couldn't substract the 'difference' from the user's userpoints.