Can a casino cap what I withdraw from a bonus?

Yes, and most do on free spins and no-deposit offers. A max cashout of 5x the bonus on a EUR 50 bonus means EUR 250 is the ceiling; if you clear the wagering with EUR 900 on the balance, EUR 650 is removed before payout. Expiry windows are the second trap. A typical bonus lives 7 to 30 days, and unmet wagering at expiry usually forfeits the bonus and everything won with it. Both figures sit in the terms, not on the banner. Read the cap and the clock before deciding whether the turnover is worth it.