YABU - she's going to be an adult - 18, not a child any more, grown up. You need to accept this.
I get that it's hard but, it's her money to squander. Her spending on things she wants is not wasting it.
What might work is asking people to write cheques, if she has to go to the bank and pay it into her account and then withdraw it, it might not disppear in the way cash in your purse does.
You could discuss what 'big thing' she'd like with her the money - maybe get her excited about a posh handbag (will there be enough for that?) or a grown up winter coat, boots, a 'forever' watch, a piece of jewellery etc. A 'thing'. Focus on how special 18th birthday gifts are compared to other birthdays and how nice it is to have something special for it.
Also, for her 18th, she'll probably not get cash in the same way, it's a special birthday, people will want to get her something special. if you start talking about gift vouchers, people will think that's what she wants (rather than what you want)/
And I repeat, she's not a child anymore, she's a grown up, if she decides to spend her money on having fun rather than buying something sensible, that's her decision.