Ctrl-F10 will release the mouse so it won't interact with the program anymore.
There is also an autolock setting in the conf.

I knew about these, but the solution, which I hadn't considered until your message - was to disable mouse in my DOS program. Silly me.

Thanks!

Makes me wonder though if it's possible to disable emulation of the mouse at the emulated hardware level, so that games that try to detect a mouse would not see one. I guess you could always boot FreeDOS and not install a mouse driver?

if you use the debugger version of dosbox
debug /nomouse