Dominick has the right idea. BLO + MS will stay good for a long time. If you don't want to buy some mason jars (they are really cheap - about $10 for a dozen with lids) then use an old pickle, mayo, jelly or whatever type of jar you've emptied from the kitchen.
If you really want to dispose of it, I'd just put it with the motor oil. Don't put it back with the un-thinned BLO, any dust or other contaminants (in addition to the MS) will then contaminate your whole container of BLO.
"Good Behavior is the last refuge of mediocrity" -- Henry S. Haskins