This is the opposite to a common DIY question, where people have a stripped or rusted screw or bolt and want to unscrew it.
In my case, I've just built a bike shed. It's held together by thin bolts that have a cross-cut head, (see image below), where the head is on the outside of the shed. I want to disable these so that it's not possible for someone to unscrew them to get into the bike shed. (I'm a bit paranoid because my last bike was locked to the front wall of the house, near the shed, and was stolen)
Ideas I've had so far are:
Using a mains-powered drill and a large HSS drill bit, drill into all of the heads to effectively strip off all, or most, of the cross cut.
Covering each head in some kind of substance which will dry rock-solid, and not fall out of the cross-cut, even after years of weather, and which can't simply be sliced away with a sharp blade.
Any thoughts or suggestions? Eg, a suitable substance for option b? I'm trying to do this project very cheaply (it's all reclaimed wood for example) so it would need to be something I can get a tube of without spending loads of money.
PS: Please don't suggest that I should have used non-unscrewable security bolts in the first place :)
Also, I would still like to be able to take it apart from the inside, by undoing the nuts from the bolts, so solutions which permanently cripple the entire bolt (eg by bending it) are not ideal.