You could employ some small loaches such as the dwarf chain loach to get rid of any snails, then simply purchase some varieties that dont breed in aquaria, like nerites, or a snail that if it breeds you can sell, like apple snails.
Just updating this thread to say I wish I'd listened to you all who said GET RID OF IT. But I was too excited to find it and I let him be...
It's about 3 weeks later now and I'm picking 3-4 tiny snails off the glass everyday when I see them. There's obviously more in there I can't see. There's not an obvious infestation/population explosion yet so hopefully I can keep on top of it.
That'll teach me to get attached to a SNAIL of all things...