What is the Sharia ruling on roommates using my belongings without my permission, and what is the sin incurred by supplicating against them for their hands to be cut off?

It appears that the girls are not negligent in using the items, as this is something established by custom. Allah the Almighty has permitted eating from a friend's house without their explicit permission due to the prevailing custom. Allah says: (Nor yourselves, that you eat from your own houses, or the houses of your fathers, or the houses of your mothers, or the houses of your brothers, or the houses of your sisters, or the houses of your fathers' brothers, or the houses of your fathers' sisters, or the houses of your mothers' brothers, or the houses of your mothers' sisters, or (from that) whereof you hold the keys, or (from the house of) a close friend of yours) [An-Nur:61]. So, if the friends act according to common custom and you assume that you would not prevent them from doing so, then there is no blame upon them. It is advisable to be tolerant in such matters, as this is a manifestation of good character. If you do not agree to this, then you must explicitly forbid them, rather than remaining silent and then supplicating against them.