Hidden in the fine print of the sign up pages on the tiny kiosk screen is an automatic renewal clause. Basically, they rent bicycles to unwary tourists and hit them with annual renewal fees long after they've left town. Even better, although you can sign up online (or at a kiosk located all around Brussels), they refuse to allow cancellation except by postal request. Tack on another couple of weeks to the process. What a shame such a nice service feels it necessary to stick it to customers in the end.

According to http://en.villo.be/Rates/Day-or-week-ticket there are 1 and 7 day tickets available.
I think the mistake is yours, not theirs.

Caveat Emptor - thanks for alerting others to what was quite legal and your fault for not reading but shame on the bike rental co for not making that noticeable.

I frankly cannot imagine how somebody can make the mistake to sign up for a year (renewable!) during a tourist visit.

Your name of "Unwary Tourist" is well chosen.

OP registered to post this -- likely won't see your responses . . .