Yes, you can. To do that, you need to enable the "show banners only" option. At the compain web page click "Traffic sources", then choose "Selection criteria":
And click a blue pencil here:
There you can enable the "show banners only" option:
You should add as many banner sizes as possible before doing this. At the time of writing this article the most popular sizes were the following:
You need at least 468*60, 300*250 and 728*90 sizes as they cover ~67% of traffic.
Mind that if you have a banner and a text ad for one ad unit size, banner will be shown by default. If you enable the "show banners only" option, all the adaptive ad units become unavailable, despite matching your banner sizes; so, the amount of traffic that you receive decreases. So, if you have all or nearly all banner sizes you probably don't need the "show banners only" option.
After enabling the option, please, check your estimated daily performance at your Budget page: the quantity of traffic will decrease and the campaign might become less inefficient. Take care!